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.