Two strap wrenches and some people use a bench vice that is heavily padded. Once you get it off just screw the new reg on by hand to make sure the threading is going in as it should. Then secure it with the strap wrench, tool force should be more then enough to hold it securely in place so no need for loctite. There should be no gap between the reg and tank body once it's on.
Just be aware of your tank/reg when unscrewing from your marker to make sure that the reg doesn't start to unthread from the tank body.