Changing my category from F2B to F1

My mother who was an LPR when she filed my I-130 is now a US citizen. How do I change the category to reflect the new preference?

You don't have to do anything until the priority date on either F1 or F2B becomes current. That's when you'll write to the office that approved your I-130 (or the state department if it's there):
If F1 is faster: that your mother, the petitioner naturalized, and that your petition should be upgraded to F1 if it hasn't already, include copy of proof of citizenship for her just in case.
If F2B is faster: that your mother, the petitioner naturalized, and you wish to opt out of the automatic upgrade and remain in F2B, include copy of proof of citizenship for her just in case.

The conversion is done by operation of law, so you don't lose anything by just doing nothing until one emerges as a clear winner. Only exception being if you marry, in case of which it'll convert to F3 and if your spouse dies or you get divorced it'll just turn back into F1 without an option to turn back into an F2B.

I'm in the same boat as you. Waiting impatiently for the next increase in filling dates. Hoping dont need to change to F1, but as the above post mentions, its simple enough, no big deal.