It should just unscrew. If it's stuck, it may have a buildup of old coffee oils clogging things up. You might try soaking it in a detergent cleaner like JoeGlo or Cafiza to dissolve the oils.

Once you get the screen off, you can remove the dispersion block with an allen wrench. It should be cleaned as well. You should also clean the group gasket and the whole group head area.

Removing and cleaning the dispersion screen and dispersion block is something that should be done regularly. I'm lazy and only do it every few weeks, but most people would probably say it should be done at least once a week. Removing all the old coffee and oils should help the flavor of your espresso, as well as removing anything that might be causing an uneven water flow from the group.

When was the last time the machine was backflushed with cleanser?