Apple warranty

For general warranty information

http://www.apple.com/legal/warranty/products/iphone-english.html

You would take it to an Apple Store or an authorized repair center, or sent it into Apple.

For the 'iPhone Service Answer Center' click here --> https://support.apple.com/iPhone/repair/service

You may wish to Check Your Service and Support Coverage by clicking here --> https://checkcoverage.apple.com/

Always go through Apple for warranty related repairs (store or online). I doesn't matter where it was purchased or by whom.

Keep in mind that the warranty is only valid in the country where the device was originally sold.

It is not international.

In most countries, the receipt is not required for warranty service.

There are some exceptions, usually those where Apple does not have a direct retail presence.