To determine if your iPhone is eligible, visit Apple’s support page. Since devices within a specific serial number range are the only ones affected, you can find out if your iPhone is one of them by entering its serial number. To find your iPhone’s serial number, go to Settings –> General –> About. Scroll down to find your serial number.
If you find out that your iPhone is eligible and is in working order, you can have the battery replaced at an Apple authorized service provider, an Apple Store, or by contacting Apple Support.
To prepare your iPhone for the battery replacement, you will need to back up your data to iTunes or iCloud, turn off ‘Find My iPhone’ and wipe your phone (Settings –> General –> Reset –> Erase All Content and Settings).
If your iPhone is eligible for a battery replacement and you’ve already paid to have it replaced, you can contact Apple for reimbursement.
The battery replacement program is currently available in the U.S. and China. It will be available worldwide from August 29, 2014.