Use the ''update-alernatives'' command

i.e. to set the default browser for the web

<code>
 sudo update-alternatives --config x-www-browser
</code>

From the man page..\\
//When using the --config option, update-alternatives will  list  all  of the  choices  for  the  link  group  of  which given name is the master alternative name. The current choice is marked with a  ’*’.   You  will then  be prompted for your choice regarding this link group.  Depending on the choice made, the link group might no longer be in auto mode. You will  need to use the --auto option in order to return to the automatic mode (or you  can  rerun  --config  and  select  the  entry  marked  as automatic).\\
Also alternatives directory - A   directory,  by  default  /etc/alternatives,  containing  the symlinks.//