I am going to teach a course, but the classes have already started with another professor. Any tips? I am the assigned professor for a course, but won't be able to teach its first two or three weeks.

The University has appointed a substitute for those weeks. Any tips on things I could do to smooth the transition? Things I should be aware of?

Thank you!
It's a hard situation. I hope the other teacher can update you on what was done. Preferably with a bit of lead time before you take over. 

I would, myself, spend the first session back doing a review. Or at least the first hour if classes are long. Ask the students a lot of questions. "What was the most important thing you learned in the past weeks?" "Anyone else? What..." 

Try to make a list of the things that they thought were important. Have them ask you a bunch of questions also. How can you help fill in as necessary? One way to get a lot of questions is to ask for them to be written out on index cards. They can be anonymous. 

This session should be mostly the students talking and you listening and prompting. Don't try to cover new material. Another trick is to ask them to write out (and hand in) sample test questions based on the previous weeks. Read these aloud as time permits. Get feedback from them on the questions. 

Your goal is twofold: "where are they" and "how can we make a smooth transition".  

This should give you a good basis for understanding where you can start in the next session. It should also give you a certain rapport with them as someone who listens. 

But you will need to be flexible about where and how you start so that the transition is smooth.