Many Stipula pens use a screw-in nib-unit.
However, you should not NEED to remove the nib for cleaning. Just pump a cleaning solution (simple: clean water, drop of Dawn [or equivalent] dish detergent, some ammonia [unless using Iron Gall ink]) back and forth through the pen a few times until it comes out clear, then flush with water.
Recently I've added a final step of filling with water (or water/ammonia mix) and standing the pen (nib down) in a cup with a wad of toilet paper, and letting it suck the liquid out. This is a slower flow than plain flushing and may pick up left-over ink that resists the fast stream from flushing. If the tissue shows a lot of color, repeat with fresh load of liquid and fresh paper.
Of course -- when I clean a pen, it means it is going into storage for some months while a different pen comes out into the working rotation.
Edited by BaronWulfraed, 11 August 2020 - 19:19.