I am just a beginner but my impression is that, for ntirates, 100 ppm isn't REALLY that bad honestly... sure it's a little high but I wouldn't call it WAYYYYYY high or anything. After all, many people have it at 40 ppm coming out of their tap...
What nitrates do you have in your tap? A water change could be a good idea.
You can clean your filter if you think it's dirty and has too much debris trapped in it, but the method that you use to clean it is important. Don't run it under tap water or anything. Just swish it or squeeze it a bit in some of the old tank water from your water change. You don't want to hurt the bacteria on your filter media!