The update can be done over the air, and addresses several known issues. Microsoft’s change log lists improvements to messaging, text-replies to reject calls, web-browsing and Wi-Fi connectivity. The update should also address random reboot problems and the ability to maintain a Wi-Fi connection when the device is asleep.
You can check for the update on your T-Mobile Windows Phone 8X by going to “Settings,” then “Phone Update.” We do not know when this update will reach the other T-Mobile Windows Phone, the Nokia Lumia 810, or Windows Phones on other carriers, but we will pass it along as soon as we find out.
source: T-Mobile USA via WPCentral