This is Apple's refund policy. It would appear you are not restricted to returning the phone to the store you bought it from though clearly that is best practice. If you are buying the X phone at the same time, that will help if you are anywhere other than the original place of purchase.
The 14 days is for most countries. Some, like the UK and the EU are 28 days. But better to get the matter resolved as soon as possible.
You will need to be able to prove the receipt copy and and personal address etc.