Hi there I have H2 on loan

I've been shooting H2/P30+... Now, the crop factor is diferent when I look through viewfinder so I want to put some mask so that I can compose the shots easier but I am not
familiar with the Hasselblad system so I thought I should ask how do you remove the Prism Finder ( what do you press?/etc. ) I see one silver button on the left but didn't press that I don't want P30+ to fall etc... better to make sure and ask!!!
So, practically I need to remove Prism Finder and put one of those P30+ stickers so I can frame the image faster and better....
Thanks