The downloaded D:OS folder is just a zip file, so will extract wherever you tell it to (usually defaulting to the zip file's current location).
If you browse to the '..\Documents\Larian Studios' folder in Windows Explorer and rename the 'Divinity Original Sin' subfolder, and you downloaded the zip file (to your Desktop, or somewhere handy), you could just right click the zip file and drag it into the Larian Studios folder before dropping; that will bring up a context menu with 'Extract...' as an option.
You do not want to delete or overwrite your current My Documents D:OS folder, since it contains all your saves (if applicable).
