The Samsung Galaxy S III for T-Mobile is receiving an update to the latest version of Android 4.1 Jelly Bean, which brings Android 4.1.2 along with many improvements including the multi-window feature which allows you to split the screen between apps.
You will receive a notification when the update is available for the device at which point you can initiate it. The update is 187MB in size, so users are recommended to downloaded it over Wi-Fi to preserve their data allowance. If you haven't received the notification, you can check for the update availability under Settings.
While T-Mobile hasn't revealed the new features which will be available with the update, users who have installed it has noticed two new apps: Paper Artist and Group Play. The handset also shows the new T-Mobile '4G' logo.
The T-Mobile variant of the Galaxy S III shares many of the features from the original Samsung Galaxy S III including the 4.8 inch display, up to 32GB of internal storage which can be expanded through microSD, an 8 megapixel camera capable of 1080p HD video capture and a secondary camera for video calls. However it will feature a dual-core processor instead of a quad-core in order to preserve its battery life.
source - TMoNews