When asking this kind of question it is very helpful if you let us know for which country. Apple's terms can vary greatly from country to country.
https://www.apple.com/legal/sales-support/applecare/applecareplus/docs/applecareplusnaen.html
"For Single-Pay Plans, your Plan Term is two (2) years (or three (3) years for Apple Watch Edition or Hermès). Apple is not obligated to renew your Single-Pay Plan. If Apple does offer to renew, Apple will determine the price and terms."
That's what Apple says. Unfortunately the supporting documentation for Applecare is still pretty much their legal documents. You may discover more by searching for "Applecare+ renewal" in the iOS Apple Support app on your device. I read one somebody else posted and it did not say you could renew in advance.