I have only just started with Zwift. Zwift has assigned me Race Group A. I do not know why. I’m not that fast at all, I see myself more at C. So of course it’s not fun to race, since I can’t keep up at A. How can I change my race class?
Thank you for your help. 
Just do a 30 minute hard ride (free ride, workout, etc) and Zwift will assign you a category.
If you go to my.zwift.com and click on My Profile, what do you see under the Fitness section? If you share screenshots of the Fitness numbers you can get more advice (it’s considered private so not viewable by others unless you choose to share it). Those numbers are compared to a set of category boundaries that you can see on the Category Enforcement FAQ page:
The point of Category Enforcement is that you can’t change it, but if there is bad data it can cause your category to become inflated. Common reasons for this might be things like sharing your account with a stronger rider, or lowering your weight in a free ride or workout.
Is my case now, i low my weight in a free ride to do a new route and now i only race in A, when i always were C. How i can resolve that now? I want to race ZRL in C as always
Email zwiftpower@zwift.com and explain what happened.
Never alter your weight in any ride if you want to enter events that use Category Enforcement.