If you don't have access to the proper spanner, you can use a pair of tweezers, a very small screwdriver, or a paperclip to unscrew the button assembly. Insert your chosen tool into one of the grooves around the button and push sideways.

There is blue thread locker on these threads, so light heat on the button may help to soften the thread locker.

There is a small red o-ring that may fall off the button threads when the button is removed. Make sure not to lose the o-ring.