Ensure that you've no remanent process runing in the background.
If you don't know how to do that through the task manager, the safest way is rebooting system.
That is:
Save your log and screenshot folder, and basically everything you want to mantain from PS to a safe folder.
Reboot system.
Uninstall the game.
Reboot system.
Reinstall the .15 version game, with the .exe
Reboot system
Download the patch, and apply it (decompress it inside PS folder overwriting files when prompted)
Reboot system
Run the updater
Launch the game.