The white washer is held in place by 3 tabs on the outer edges.
My approach was to push the piston in (master cylinder disconnected from hose), and with a small flat screw driver pop the washer out by a slight prying pressure.
Notes:
1) if you snap the washer, you will need the rebuild kit or a new part.
2) If you goto magura to get a rebuild it, its much cheaper than ktm.