See the following from Extend your AppleCare+ coverage - Apple Support
If you paid upfront for an AppleCare+ plan, you might be able to purchase new coverage within 45 days after your original coverage ends. In China mainland, you might be able to continue coverage within 30 days of the end date of your coverage. Not all products and buying options are available in all countries or regions.
Also see the following from that same support article:
Extend your coverage
- On your iPhone, iPad, or Apple Vision Pro, go to Settings > General > AppleCare & Warranty, then tap your device.
- On your Mac, click the Apple menu  in the corner of your screen. Choose System Settings > General > AppleCare & Warranty. If you can’t find System Settings, choose About this Mac, then choose Support.
- Go to mysupport.apple.com and follow the onscreen instructions.