Phone Repairs

My Phone has stopped working completely. I've tried all the restarts, DFU, charging, etc. with no luck. I don't have a local shop to drop my phone off to be repaired or have looked at but I am covered under warranty, what is my best option. I would have assumed it would be possible to send my phone in i.e. by post, but struggling to find any details for this, could anyone please advise?