First, a couple of tips:-
(1) Even if, like me, you have only 2 or 3 coffees a day, backflush your group at least weekly, using Urnex Cafiza or something similar – you’d be surprised at how much crap is hidden on the top side of the dispersion screen, out of sight – I certainly was when I removed it. If you have a suitable screwdriver (No. 2 Posi – and do use the correct size, the metal of the screw is quite soft and easily damaged), take it off and see for yourself – then give it a scrub. When you replace the screw, put a little food-grade lube on the thread, stop it seizing. You will, of course, need a 58mm blind filter also called a blanking disc, for back-flushing (i.e. one without holes). That link also has other sizes, plus downloadable instructions if you don’t already know how.