The Galaxy S6 at C Spire, whose model no. SM-G920T is same as that of T-Mobile S6, is receiving the Marshmallow update today. The Android 6.0 update carries the build no. G920TUES4DPF1, PF1 in short.
The OTA is already out. You should check into Settings and look for a system update right away, and you should get one. Though, under the phased release, it could take a few days.
We don’t have the changelog with us right now, but when C Spire brings it out, this article will updated to reflect that.
The update should also bring monthly security patches along with the Marshmallow OS. if you get the update, do let us know what else of the changes you are able to spot.