Renew Apple Care
I bought the one year plan, how can I renew it, or change to an ongoing monthly plan.
No such thing as a 1 year plan for Apple Care+.
It's either 2 years for iOS and iPad OS devices, or 3 years for Macs.
If you bought the 3 year option for Macs, you can extend it into Yearly plan once it expires, if you are in one of the countries that supports the extension.
See here: Link-> Extend your AppleCare+ coverage - Apple Support