The .desktop files is something like the launcher where is saved under /usr/share/applications, you can use any ASCII editor to modify it or there is anohter GUI way.
sudo apt-get install gnome-panel
Use the following command to create a new .desktop under /usr/share/applications
sudo gnome-desktop-item-edit /usr/share/applications/ --create-new
Also put the exact .desktop and full path without –create-new can edit current .desktop files.