A 177MB sized upgrade is reaching Samsung Galaxy S6 and Galaxy S6 edge on T Mobile, using an incredibly common change-log suggesting that there are performance improvements, bug fixes and new/enhanced features. As soon as a user will start using the phone, it will jump to build number UVU1AOCG. Currently we can not notice any user-facing changes to the upgrade, thus this is likely all behind-the-scenes things.
We are happy to see T-Mobile and Samsung pulling the trigger on improvements and fixes before most folks will reach to the bugs.