There's a spring loaded detent that you should see at the bottom of the Magazine Catch Nut, some are to the side but the 5906 should be at the bottom IIRC? you will need to push it in with a dental pick or something similar & then thread the nut of counter clockwise..
Reinstall in reverse.. The detent has a spring beneath it, don't loose these two small parts..
I have a screw driver bit that the center is gone just for these if it's tight.. Try putting a drop of light oil on the center of the button beforehand such as Rem-Oil or WD-40..
You can probobly turn it with your thumb by applying preasure & supporting the other side while depressing the detent..
Good Luck!!
Gary/Hk