Want to extend warranty

It’s too late to extend it if all you have is the basic 1 year warranty. You have 60 days after purchase to add AppleCare+ to your phone.

You can't.

AppleCare+ can only be added within 60 days of originally purchasing the device.

No. If you don’t purchase the extended warranty when you buy a phone you can’t add it at the end of the warranty, just like for car extended warranties.