Can't prepare lesson material. Should I cancel class?
I would like to add on top of the answers of @aparente001 and @Anonymous Mathematician.

So, first thing, in 5 hours it would be very hard to come up with something solid that you would be comfortable lecturing about; thus I second the suggestion of doing some review and exercises for now.

My opinion is: you can do it.

If you have been teaching calculus so far, chances are that you are a very competent student. You can learn almost at the same time as you will be teaching. And this does not have to be a stressful experience, I would say you can be upfront with your students and let them know that you are learning at the same time as them (they already know that you are a student too).

Actually, this can be beneficial for both parts: you) are learning a new topic and because of that you better understand the difficulties that students may come upon; they) have a TA that understands their difficulties and that is able to explain things in a way that a student would learn (it may not be the best way, but I would say that is OK).

In the case they make you questions you do not know how to answer, again, be upfront: say that it is a good question, you will think and research about it, and you will explain it later, either in the class, e-mail directly to the student, whatever you prefer.

In order for this to work, like mentioned in the other answers, you have to talk to your supervisor and let him know that you were not prepared to lecture this topic. Ask him for help about learning those concepts, or a reference to whom could provide such guidance.