It is a good practice to clean the inside of the shower screen everyday in busy coffee shop but if you have one of these commercial group at home you probably needs to so it really once a week, as long as you do use a brush to clean the screen every time after use.
The way spring lever machine works makes the shower screen and brass block cleaner than pump machine as no liquid back flushes, water always goes down.
To remove screen, use a flat head screw driver, locate it at the recessed line of the screen, then use the edge of group as pivot, then lift. Do apply same action on both side.
If you are worry scratching the group’s edge, do protect it by a tea towel.
Once it is off, do remove the rubber but make sure you know which side faces up ( a slight different profile ).
To assemble it back, simply put it into the portafilter then lock it back in!
(Thanks to Pokit from HK and Roberto from Bosco for the clear instruction )