If you get a menu that says Modify, Repair or Remove when opening Final Draft, like so:
Then you have opened the FinalDraftSetup installer file, instead of Final Draft 13 itself. Final Draft 13 is already installed on your PC if you are seeing this window.
To open Final Draft 13, go to your Start menu, which is the Windows icon in the bottom left of your screen, then either click All Apps in the top right or search for Final Draft 13, and click the green and black icon that appears.