Assuming you are running some version of Windows, you want to do an "Uninstall"

In Windows you can go to your "Control Panel" and then click on the 'Programs'/Uninstall a program'. This will give you a list of software that is on your computer, it should be alphabetical. Just go down to where you see "Gimp" and select that. The uninstall program will then walk you through the removal.

After all is completed you can re-download and install Gimp again.

You should only do this however if you can't use Gimp at all. Honestly, as far as I know they don't have an 'online' version so I'm not sure what you are dealing with. If you can open Gimp and start a new drawing and draw then I wouldn't do the uninstall. I would just pull up the Gimp manual in a browser and then get busy. I don't think I ever used those links in the help menu.