You'll need to use a text editor to change the file at that path.

The editor that I know is on there by default is vim. so in this case you would type (after the sudo gainroot)

vi /home/user/.osso/mis/internet-search-applet.xml

If you're not familiar with vi or vim you may want to google some of the common editing commands. The main commands in this case would be dd to delete a line and :wq to save the file and quit.

HTH