Have a look to the man of update-alternative command.
Once you've added your alternative, you will end up with something like that :
$ update-alternatives --display x-www-browser
x-www-browser - status is auto.
link currently points to /usr/bin/firefox
/usr/bin/konqueror - priority 100
/usr/bin/firefox - priority 110
Current `best' version is /usr/bin/firefox.
You must add a higher priority to your alternative