139 seems wide. 135mm is the widest I know and it is very common on mountain bikes. Measure it like this.

Regardless of the actual spacing, changing it is very simple. Just remove the outer nuts and place spacers on the hub axle. If your wheel is dished symmetrically then just put 7.5mm of spacers on each side to go from 120 to 135mm.

But... If you have a mtb frame with an OS 73mm BB shell, you will have chainline problems if your hub has a 42mm chainline (most likely).