Apple will replace the sleep/wake button mechanism, free of charge, on iPhone 5 models that exhibit this issue and have a qualifying serial number.
If your sleep/wake button isn’t working properly, you can enter your iPhone 5 serial number at the link below to see if it qualifies for the program. As of April 25, 2014, the replacement process will be available in the U.S. and Canada. In other countries, it will be available as of May 2, 2014.
Replacement of the sleep/wake button mechanism is done at an Apple Repair Center. There are two ways to send your iPhone to the repair center—carry-in or mail-in. The replacement process takes approximately 4-6 days from the time your iPhone is received at the repair center until it is returned to you.
More details as well as instructions on how to send in you device can be found at the link below...