If you are mechanically inclined you should be able to do it without issue as long as you have a pressure tester and there are both bellevilles and a couple of shims in it.
1) Degas bottle
2) Remove bonnet set screws and then remove bonnet.
3) Remove piston and remove one of the shims. If different thickness start with thinnest one
4)Reassemble - pressurize and check pressure
5)If pressure is still too high repeat process
You may have to purchase some really thin shims for fine adjustment
Look up Hajimoto videos, he has one on the complete process of setting up the desired pressure on a Ninja regulator, including a complete list of everything you need to do the job. He even includes the parts required for the pressure tester.