This thread should be in the main group and not Hypertherm.
The Mustang with a B5 CNC used a drive system that was made by C&G.
They mounted the resolvers in the gear rack a few inches ehind the drive motors.
It has been a while since I worked on one, but here goes
Remove both drive motors from the gear rack.
Have someone hold the drive power button on (It may be a toggle switch)
Pull the resolver out of the gear rack, and slowly rotate the resolver gear until the alarm clears and the sync light goes out.
Carefully put the resolver back in the rack, then the drive motors.
It may take a few tries but that is the simpelest way.
When you square the machine, you can rotate the resolver housing to get the machine square. Loosten the 3 screws that hold the resolver to the frame, and rotate it. You will see one side of the machine move when you rotate the resolver. Get it as square as possile.