If you are looking for options to zumba, either to enter back as an instructor, or for your own fitness, there are things you can consider. Have you had a full physical to look at your knees and get some feedback on what activities are helpful and which harmful, are they arthritic, do you have old injuries. Knowing what your baseline is will help with either question.
Depending on what is going on with your knees you could try silver sneakers, or Tai chi, or chair yoga, or in fact the adaptive form of any type of exercise. You could also try transitioning to water aerobics.
If you are comfortable doing the format yourself, and if your medical advisor does not advise against that type of exercise then it is a relatively simple thing to find out what the requirements are to begin teaching again. I suspect if you haven’t been teaching you probably havent been doing your renewals and CECs, so you would likely have to sit for the exam again.
best of luck