Can't switch Applecare+ from monthly to yearly

I bought an iPhone 13 last month from an Apple Store, and I was planning on buying at least a year worth of Applecare+, but the person who worked there who sold it to me suggested that I sign up for a monthly plan, and then switch to yearly after that, due to something or other involving showing the date of purchase on the phone, or something like that. When I go into the settings on the phone, it only gives the option to cancel or renew the monthly subscription, and online, it simply tells me that the phone is already covered. So, how do I switch?