I can't update my unlocked iPhone 4 to ios 6?

Is it because I'm t-mobile?
my phone is factory unlocked and last time my phone let me updated it to 5.1.1:/ now it is not letting me update to 6.help? I even tried it through itunes and I still got the same thing. I'm also on the latest firmware.

I don't know I'm haveing a bad time getting ios on my phone

It's probably because you do not have enough storage space.