The update includes support for LTE bands 2 and 5 and the update brings an FM radio to most Windows Phone models. To use it, you will need to plug in your earphones. Sync support has been added to Google contacts and calendar and additional enhancements have been made to the performance of Bluetooth, music and the battery.
The update also includes the Data Sense application which keeps track of your data usage each month. You can see how many days are left in the billing cycle and even find out which apps are consuming the majority of your monthly allowance.
AT&T says it is sending out the GDR2 update for the HTC Windows Phone 8X as a staggered release which means that it might be a few days before it hits your phone. No need to panic, the update will eventually reach you!
source: AT&T via WMPoweruser