T-Mobile is going to tell you to watch that link for update information. Probably also Software versions & updates: LG K7.
T-Mobile will not announce software updates outside unless it's in one of those two locations (and if it's in the first, they won't give a when it will be released until it's released).
My personal assumption is it probably will get Marshmallow since it was released this year. Based on the way LG does things, possibly by summer.
Manage your T-Mobile account