How to Deal With a Threat
To deal with a threat, start by gauging the urgency of the threat. If the threat is more abstract and less immediate, like blackmail, try to get a clear picture of why you're being threatened. If you know the person who is threatening you, ask if you can resolve the situation in some way without escalating it any further. For more immediate threats, check around you for potential defense mechanisms and escape routes. If you have no choice but to physically engage, kick the person in the groin, elbow them in the ribs, or punch them. You can use the moments immediately after that to make your escape. Be sure to contact the authorities as soon as possible to describe your attacker and the location of the incident.
Gauge the urgency of the threat. Decide how certain you are that the threatening person will follow through on his or her words. There is a wide margin between a threatening note and a man standing in front of you holding a knife. The way that you react will necessarily depend upon the immediate danger of the situation. Evaluate the situation. If the threat is immediate, then quickly and calmly look around you for potential defenses and escape routes. If the threat is more abstract, then try to get a clearer picture of what exactly is going on. Make sure that you understand why you're being threatened and what the actual risk is. Why are you being threatened? If you don't know, ask. If you can't ask, then speculate. Do they want something from you? Consider giving a threatening person what he/she asks for. You never know how desperate someone is, and it is senseless to be killed over the contents of your wallet. Who is the leader of the group? If it comes down to you against them, then the leader may be your first target. Assess the area. Are you familiar with the lay of the land? Are you under CCTV surveillance? Do you have a chance of escape? This will strongly define the actions that you take to deal with the situation. Talk to the person. If you personally know the person who is threatening you, see if there's a way to resolve the situation without escalating it any further. Strike a deal, if you're being blackmailed or asked for something. Discuss the situation face-to-face and try to come to a mutual agreement. Figure out whether you're being threatened for something valid. Perhaps the person thinks that you did something that you didn't do. Don't be too proud to apologize. A good apology can defuse some of the tensest situations. Deal with blackmail. Blackmail is still a threat, even if it is nonviolent. The way that you respond will depend upon what the person is holding over you and how much you have to lose. Make sure that you don't give in before you've considered other ways out. If you feel confident, take a stand. Tell someone. Make sure that you aren't dealing with this alone. As soon as possible, involve someone that you trust: a teacher, a parent, a friend, a partner, a coworker, an authority figure. Together, you stand a better chance. Show the person any threatening messages, and make sure that he/she knows exactly who is threatening you. Get a restraining order. If there is no other way to defuse the threat, then consider obtaining a court order for the person to stop bothering you. You'll need to prove the truth and urgency of the threat, and file a report with the local police department. You can ask for an order for the person to stop specific behaviors, or you can ask that the courts restrict the person from coming near you. Once you have a stay-away order against a person, he or she cannot come within a certain physical range of you – often on the order of 50-100 yards. This may not stop the threat if the person is desperate enough, but it will at least create a legal barrier. Act nonviolently when possible. Try to handle the threat by giving in, or escaping, or talking your way out of the situation. You may find that people are much more reasonable than you expect. Decide whether you have an escape route. If they are only facing you, then you might run backwards. Run towards other people, there is safety in numbers. If there is no nonviolent way to escape, then you may need to defend yourself. Be prepared for this, but try not to make it your first reaction. Defend yourself. Be realistic about your chances. If you are outnumbered or otherwise outmatched, then it might be wise to first explore nonviolent solutions. Remember that violence is never a guaranteed method of dealing with someone. Once you escalate the situation, it may be very difficult to safely cool it back down. If you are under CCTV surveillance and you plan to fight your way out of the situation, then you want the threatening person to make the first move. However, if you are outnumbered, and one or more of them is armed in some visible way, then this may be enough to justify your actions. Take down the leader. Try a kick to the groin, an elbow to the ribs, or a well-placed punch. No points for style or fair play – but if you put all your strength into it, he/she should hit the deck fast. Now you need to think again. Run now, if possible. Move quickly away through the space you just created. If you're lucky, the rest of the group will be momentarily distracted. If you can't get away, then you need to get something in between you and the rest of the group. One of the group will do nicely. Grab the closest one by the throat or neck—you want to be behind him so he is not in a good position—and make sure that you are hurting him enough that he won't attack you. Try grabbing his ear with your other arm and pulling it as hard as you can. Fight for it. Keep the fighting close and dirty. Dart in and out again, and don't let them grab you. You're finished once one or two of them catch hold of you. Run as soon as you get an opening. Kick the back of your "shield's" knees down to the floor in a very powerful stamp. Ideally you want to break something. You then need to deal with the others in a similar way. Try to go for unexpected targets. The knee is very weak, and can easily be broken with a kick. Sucker punches to the jaw can lay someone out, but they are so very expected that it's unlikely to come off. Contact the authorities. Tell the police or a security guard about the confrontation. Alternatively, use your cellphone or a payphone and call the emergency services. Do your best to accurately describe the situation: when, where, and what the threatening people looked like. 
Gauge the urgency of the threat. Evaluate the situation. Assess the area. Talk to the person. Deal with blackmail. Tell someone. Get a restraining order. Act nonviolently when possible. Defend yourself. Take down the leader. Fight for it. Contact the authorities. 