How should i buy apple warranty

you cannot "upgrade" your warranty, but you can extend your Applecare + plan on your phone

https://support.apple.com/en-us/HT210580

You must buy it initially within 60 days of purchasing the phone.

If you do not have Applecare, your factory warranty cannot be extended.

sukhveer274 Said:

"How should i buy apple warranty"

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Some Things to Point Out:

For the instructions of how to do so, refer to the link provided to this post.

As for how to pay for it, that's your call.