How to update monthly plan to annually

you'll need to cancel and restart.  make sure you have your cc files downloaded in case there's an issue.

then contact support to make the change (which you can do now without a penalty).  there are 2 ways to contact adobe support; chat and twitter (now x):

chat:
if you have or want to purchase a subscription, use a browser that allows popups and cookies
and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

twitter/x:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative.