It doesn't need to update through the Microsoft Store. Did you go to settings->phone update->tap check for updates ? If there's no update, you might already have it installed.
Check out this doc from T-Mobile about updates:
https://support.t-mobile.com/docs/DOC-5828
It also tells you how to check the version you have. Can you please try those steps, and also share the version that you have with us?
There's an update that you have to install before you can get to 8.1.