buying cpo s5 from t-mobile with kit kat - how do I know if I can update OS?

[AC Question]

I'm thinking of buying a CPO S5 G900T from T-Mobile that only comes with kit kat (4.4.2). What do I need to know about updating OS to 5.0 or 5.1? Are all G900T's capable of being updated? I don't understand why they don't update the OS when they re-furbish it - thus my concern.
thanks