Works the same as hardening the pivot pin holes on an AK receiver. Cherry red, dunk it. Draw it back out to blue. Done.
That said, if the spring isn't working now, chances are it won't after re-hardening unless proper steel was used with improper hardening in the first place. I've seen a lot of cheap chrome "springs" that weren't worth spit.
Also, each heating drives out some of the carbon necessary to the steel's spring properties. You can reach a point where all you have is mild junk steel.