On the other part of the cartridge undo the screw on the right hand side and put aside carefully. Then you can lift the wiper from its seat until you can release it from the tit at the left hand side and remove it. Wipe it down and replace it.

The second picture shows clearly the mark made by the screw that fell into the mechanism. I used a soft paintbrush to clean the ink roller. There might be better ways.

To refit the wiper start at the left side and slowly feed it down close to the roller. I used a small screwdriver to help to position it as towards the right side it started to curl up.