The easiest way is by removing the group by undoing the two bolts which hold it there. It’s dead simple. Just be careful when removing it as the syphon pipe is attached to the group.
Before removing the group, remove the lever.
With the group out of the body of the machine, and sitting on a flat, strong wooden surface (i.e.: table) you can then use a rubber mallet to hit the top of the rod. Alternatively, with the group upside down in your hands, press the rod against a flat surface.
The only thing holding the screen against the group is dried up coffee, btw. :-) - As you will see, it feels like it’s cemented. Another reason to do this process every 6 months or so. If you keep it clean, everything comes off a lot easier.