Just run the installer, that's it. There's only one installer, which is a small file that downloads the bulk of the files and starts the installation.
To SAVE you from downloading the bulk of the files again, you have been offered to SAVE them in your Documents\GSX_Installer folder so, if you took that chance and asked them to be saved, you should have a copy of the files and, since there's just one .EXE there, there's no doubt what you need to start.
Of course, you don't say which simulator you use, so there's one installer for 32 bit simulators ( FSX and P3D V3 ), and another for 64 bit simulators ( P3D 4/5 ), but the principle is the same.
