1) Download the v1.3 update from
here. Run the update file and if automatic detection & installation fails I suggest you select your desktop as install directory like this:
2) At the next screen, remove the tick to create Start Menu directories and proceed. When finished, remove the tick to start the game.
3) Then go to desktop folder named Amnesia - The Dark Descent. Go to redist folder and cut-paste those files at the Amnesia directory for a manual installation. If done correctly, it should ask you to replace a lot of files including Amnesia.exe etc.
4) You can now delete the folder at the desktop as well as the update exe file.