This is just "thinking out loud" as I don't have the exact
syntax of the code available, only the "theory behind the thought"

First -> find out which folder of your system the executable
is stored (maybe it's /usr/bin or /usr/lib)...

Then, use synaptic or command line to completely remove gnochm.
--- BEFORE YOU DO, pleas see below @ "Why this might not work"

Once gnochm is removed, you can then create a symlink (or soft link) that
if "gnochm" is requested , point instead to kchm in the same folder where the
gnochm executable is or was.

Why this might not work. Once gnochm is removed, it may (possibly)
disappear from your Preferred Applications list as a chm file handler.

Possible workaround... once you do find the executable for gnochm,
then (as sudo-root) rename the file. This should prevent gnochm from launching
under its given/natural name.

Since the system still knows of gnochm, you can then create the symlink.

Again, I don't know if it will work or be the most practical solution.

Thanks, but this seems too complex for me.
What I do now is to right click and choose kchm. I was hoping for a way to make
it the default, like you can do in Windows when you want to make a certain
application the default for opening such file types.

I use xchm from the repositories which seems to work fine for me. The
easiest way to change the default application that I have found (there
may be an easier way!) is to locate _any_ of your chm files, right click
on it and select "Properties". In the tabbed dialogue box which will
appear, select "Open With" and click on the radio button for kchm. Close
the dialogue. This will make kchm the default for all your chm files.