Question Got 4.5.81, want to update it but can't?

A buddy gave me a phone he's had laying around -- don't know where he got it, but it's got 4.5.81. No, I didn't mis-type. Since I don't think it was ever released (trial image or something?), it won't pull an OTA update.

I'd like to keep it as a stock test phone, and leave my own Atrix running CM9. How do I go about updating the .81 phone to, say .83 (or newer) so that it will pull down the latest OTA updates. I'd like to not have to unlock the phone if possible-- want this one to be "stock".

Any ideas??