Hi henricius, there are a couple of different ways to fix something like that. One would be to extract all the curves which is done by selecting the edges and using copy/paste to duplicate them as regular curves, fix up the curves and build a new extrusion from the improved curves.

But another way that's a bit more "surgical" using some more advanced low level tools would be something like this:

Select the tiny face and the smallish one nearby it as well:



Push delete to remove those faces leaving a hole like this:



Select these 2 edges:



And run Construct > Blend to build a smooth blend surface between them, it will look like this:



So now there's a little bit of extra material on the top and bottom faces that's sticking out and needs to be removed. You can do that by selecting the top face, then run the Edit > Trim command, then select the top edge of the blend surface as the cutting object. That will cut the top face and then you want to pick this fragment as the piece to remove:



Once you have trimmed out that area from both the top and bottom faces you'll now have a repaired set of surfaces which all match up, select them all and run Edit > Join to glue them back into a solid:



- Michael