Espressione Cafe Retro

I'm trying to replace the group gasket on my Espressione Cafe Retro machine can someone tell me how that is done. looked online and it said that a screw needs to be removed underneath the filter. I have the gasket and I see where it is supposed to go, I just can't figure out how to remove the old gasket and install the new gasket. There is a round fitting in the center of the filter with four holes on it. I tried turning it but it doesn't budge.
Thanks for any advice
Ron