Help guys- update problem

Hi everyone. I have a problem to update my xt890. i've just bought second hand Motorola as a spare phone. Currently running original 4.1.2 Jelly Bean. when i'm gonna to check updates, this option " Software update"under About remains not active (grey color) it doesn't respond by tap. I've done a factory restore but it's the same.
Phone bought from first owner (older lady) and never has been messed up. Any suggestions?