If you're dealing with a shredder that has fully seized up, there's two locations it will be stuck and this is one of them.

Remove the 4xH5 22mm Screws from the end.

If the shredder is siezed, spray in the marked locations with some penetrating spray and give it a few minutes to soak in.

Take a big hammer and tap on alternating sides to try get the end cap off. The amount of force you need will vary from "just a light tap" to "am I going to break this?" depending on how rusted in it is.

The first one I did had been sitting around for months in the weather and took me a lot of hammering to get off. The second one (had been seized for ~a month) was just a couple taps and it just popped off.

Once you get the end off you can take out the adjustment piece - note there's a plastic spacer and a nut that goes onto the red adjustment knob, put these aside so you don't lose them!