About the warrenty

Warranty is 1 year and 1 year only.

If you bought Apple Care+ it extends the warranty to a second year. You can buy Apple Care+ within 60 days of purchasing your device..

If your Apple Care+ plan has not expired, you can change it to a monthly payment to extend it beyond the 2 years the one time purchase of Apple Care+ provides.

See here: How to manage your AppleCare plan - Apple Support

Are you currently using AppleCare +? And when does it end on your iPhone?

About continuing your AppleCare+ coverage for iPhone, iPad, and Apple Watch - Apple Support

You can't extend the warranty.

Yes you can extend AppleCare +. Read the link I posted.