The screw they talk about is simply you screwing the clear plastic top off by hand.
You might not even need tools to remove it to an easier to work location. Most of the more modern Jabsco filters are attached to the hoses using a bayonet type plastic fitting each side of the filter. Ours are both pump and hose side.
If you'rs are the same then this simply push those bayonets open to wiggle the complete assembly out. I think once you have up on a bench top you'll find it a lot easier to unscrew by hand.
BTW when reassembling we always use a little petroleum jelly to lube the O ring / screw and ensure the seal is watertight.
Some some ney-sayer may appear and tell you petroleum jelly could rot
the rubber O ring, but my own experience over 5 years doesn't support that.
But the greasing will also make it easier also to remove next time..............