Looking at installing Huma Reg on FX Royale 500, how to adjust hammer spring?

Use a butane "hobby" torch to apply heat around the nut for about 10 seconds. With the Allen key in apply pressure while heating until it breaks free.
most people here use vibra- tight thread locker so you can easily readjust it again. 
I took a different approach and ordered an extra hammer spring adjustment nut and put that behind the original one to act as a jam nut. It's been holding in place just fine so far. 
Good luck
Dan

A hair drier will work as well.

I like the hair dryer idea that Tom mentioned. I've also seen guys use a soldering iron held directly on the nut to free up the lock tight. Only the nut gets hot, nothing else. I like that idea the best of all.

All too often guys hold their butane torche's flames a bit to long and overheat and damage the bluing or some adjacent structure. Can't happen with a soldering iron.

home depot sells heat guns reasonably cheep they get hotter than hair dryers ,I use them for heat shrink on electric wires heating and bending pvc etc, a little safer than a torch but be careful they do get hot!

Make sure you use the right size hex! 5mm, and if you want just roll up a piece of plumber's putty and smush it between the adjuster and into the threads when you find where you want it set it never dries and itll keep it in place just fine.