in kmeleon's folder look for a binary called setdefault.exe and run it, you can also launch it from preferences>browsing>make default button
if you fiddle with bad browsers(stealing defaultry without asking user) a lot, then you might like to try km defaulter which automatically checks if km is default and prompts to make it default if otherwise
http://kmext.sourceforge.net/files/kmdefaulter.7z
kmdefaulter.exe must be saved in kmeleons root folder(same path as k-meleon.exe) create a shortcut to kmdefaulter on your desktop/startmenu etc for easy access.