How to Remove Henna from Hair
To remove henna from your hair, first get a large bottle of mineral oil or make your own solution from olive, argan, and coconut oils. Then, massage the oil from your roots to your tips and along your scalp until your hair is dripping with oil. Next, cover your hair with plastic wrap or a plastic shower cap, and leave it covered for at least 2-3 hours. Finally, shampoo your hair with a clarifying shampoo to get the oil out of your hair. If oil doesn't work, you can try applying alcohol or something acidic, like lemon juice or honey, to strip the henna from your hair.
Buy a large bottle of oil. Anecdotal evidence supports using mineral oil over other types of oil, but you could try a combination and see what works best for your hair. You may be able to make your own oil solution out of olive, argan, and coconut oils. You will likely need several applications, so be sure you buy enough to cover your head several times. Soak your hair with oil. Stand in the bathtub or outside and massage the oil from the roots to the tips and along your scalp. After you have covered your hair completely, place more oil in your palms and massage it through your hair again to be sure it is completely saturated. Oil should be dripping off of your hair. If it is not dripping a little, you have not oiled it enough, and you should apply more. Cover your oiled hair with plastic. You can use a plastic shower cap or just plastic wrap from the kitchen. Cover your hair with plastic to keep the oil wet and let it soak in to your hair. Heat your oily hair. This optional step may yield better results. Use a hair dryer to apply heat to your oily hair.  You could also go out into the sun if it is a warm day. Leave the oil on your hair. The longer you leave the oil on your hair, the more it will help lighten the henna color. At a minimum, the oil should be left on for 2-3 hours. Leaving the oil on overnight may be beneficial. If you leave the oil on overnight, cover your pillow with a towel so that it will not get oily if the plastic moves in your sleep. Testing showed that a 12-hour application of oil had significantly greater impact on hair tint than 2-3 hour applications. Shampoo your hair with a clarifying shampoo. Use a high-powered clarifying shampoo to strip the oil out of your hair. Try massaging the first application of shampoo over the oil before wetting your hair with water, then rinsing well. Lather with shampoo and rinse several times, until your hair no longer feels oily. Depending on the oil, the shampoo, and the hardness of your water, this could take many repetitions to get the oil out. Use water that is very hot, and be sure to rinse thoroughly. Repeat the oiling process. Multiple applications of oil are more likely to make a bigger difference over time, so repeating the process several times will give you better results. Give yourself a week between oilings to give your hair a chance to recover a little and produce its own natural moisture. Try other products. Other products that may help fade or lighten the henna out of your hair are grape seed oil and whitening toothpaste. You can try these products with the same process as mineral oil. Apply alcohol to hair. This method may be best when followed immediately by an oil treatment.  It may strip some of the henna and prepare the hair to absorb oil better, which will make the effects of the oil fading stronger. Squeeze lemon juice onto your hair. The acid in lemon juice, particularly when combined with natural sunlight, can help strip the henna off of your hair and lighten the effects of the henna. Freshly squeezed lemons will yield better results than concentrated lemon juice. Combine lemon juice with water and dip your head into the mixture to coat your hair completely. Go outside in the direct sunshine and let your hair dry. You may want to “fluff” it with your hand periodically to be sure that multiple layers of hair are receiving the benefit of the sunlight. Be sure to moisturize your hair or do a deep conditioning treatment after applying acidic products. Coat your hair with raw honey. Even though it tastes sweet, honey has some acidic properties and may help strip henna out of your hair without damaging it as much as harsher, more acidic strippers. Leave the honey on for at least 3 hours, longer for better results. Be sure to wash thoroughly with a clarifying shampoo afterwards. You may want to stay inside while letting your hair soak so that you do not attract bugs or bees to your head. Use peroxide. This option is not ideal because it can severely damage your hair, but if you are desperate to get rid of the henna, you might want to try this as a last resort. Coat your hair with hydrogen peroxide. Be sure not to get it in your eyes. Leave the peroxide on for an hour. Apply heat by going out in the sun or blow drying your hair on high heat. Wash your hair with a clarifying shampoo. Do a deep conditioning or hot wax hair treatment to help salvage your damaged hair. Use a non-oxidizing dye to cover the henna. Hair dye that does not contain peroxide may be able to cover the henna better than traditional dyes, which may cause the hair to turn blue. Grow the henna out. Growing the color out of your hair may take a long time, but it might be the best option if you have dyed your hair with henna. Trying new hairstyles while your hair is growing may help you pass the time during this frustrating period of time. Cut your hair very short. If you cut your hair short, you might be able to cut off most or all of the henna-dyed hair. Even if the henna goes all the way to the roots, short haircuts will grow the color out sooner than long hair cuts because there is less area to grow out. Wear a hat or a wig. As a last resort, try wearing a hat or even a wig until your hair has recovered from the henna treatment. 
Buy a large bottle of oil. Soak your hair with oil. Cover your oiled hair with plastic. Heat your oily hair. Leave the oil on your hair. Shampoo your hair with a clarifying shampoo. Repeat the oiling process. Try other products. Apply alcohol to hair. Squeeze lemon juice onto your hair. Coat your hair with raw honey. Use peroxide. Use a non-oxidizing dye to cover the henna. Grow the henna out. Cut your hair very short. Wear a hat or a wig. 