T-Mobile has begun rolling out a new update for the Samsung Galaxy S6 and Galaxy S6 Edge containing the December and January security patches, and seemingly not much else. T-Mobile lists the software for each phone as having a release date of February 6, which means that some users will have already gotten the updates, while others may see the updates coming in the next few days or so. T-Mobile's website lists the changes for the updates as being limited to the December and January Android Security patches, with no other changes. The updates' small sizes corroborate this, with one user who has already gotten their update putting the size at a mere 23.87 megabytes.
The December and January updates don't quite put Galaxy S6 and Galaxy S6 Edge owners in line with Pixel and Nexus owners, who recently got the February patch, but they do bring a number of vital security fixes for some nasty privilege escalation exploits that have been discovered recently. The update will bring the software version number up to G920TUVS5EQA2 for Galaxy S6 owners, while those rocking a Galaxy S6 Edge will see software version G925TUVS5EQA2 after the update. Despite the update's small size, all the usual caveats apply; you'll have to be on stock firmware to catch the OTA update, and you'll want to have your battery charged to at least 50%, to avoid any issues with applying the update.
Major updates for the Galaxy S6 and Galaxy S6 Edge have mostly hit a wall at this point, but the phones are expected to receive an update to Android 7.0 (Nougat) eventually. The last time these two phones were updated on Magenta was back in the end of November 2016, when they got the November security patch and some improvements to their roaming performance. T-Mobile is doing a fairly good job of keeping the devices up to date on the latest security patches in order to ensure that owners' data and devices are protected, but there is thus far no word on exactly when the update to Android Nougat (something that even Galaxy S7 owners are still waiting on) will drop.