I have a Nokia Lumia 925 and a T-mobile Select Choice plan with 3GB data service.  Unhappily, the 925 came with built-in carrier AT&T.  Even though my T-mobile SIM works fine, I am unable to activate internet sharing.  It responds with "current data plan doesn't permit internet data sharing."  What do I do to flash the bios or whatever it's called to change my carrier to T-mobile?

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