How do I remove a shower screen from my brewing group?

This does depends on what model you have. Some brands use shower screens which are held in place with the portafilter gasket, but you have also models which are held in place with a screw/nut.

With the friction fit shower screens you first need to remove the portafilter gasket before removing the shower screen. The disadvantage of this is that if you want to clean the shower screen on the inside you likely also ruin the portafilter gasket.

For removing a shower screen with a screw, make sure that you use a good screwdriver. A good screwdriver is a lot cheaper then using a incorrect one, which ruins the heads of the screws. There are 2 models, a Wera Philips head screwdriver and a Wera slotted screwdriver