How can I renew my Apple care plan?

Read this document:

About continuing your AppleCare+ coverage for iPhone, iPad, and Apple Watch - https://support.apple.com/HT210580 - "If you paid upfront for 24 or 36 months of coverage, you can renew coverage on a monthly basis after the 24 or 36 months is over."

Here is a support article→Extend your AppleCare+ coverage - Apple Support

Not that you can’t extend it until the day after it expires.