While often listed on various sites under "free download" titles, the official way to obtain for free is specifically for owners of the original base game:
"Update 5" generally refers to one of the final major patches for the Unrated edition. These updates typically focused on: Agony UNRATED Free Download (Update 5)
: Upon launching the game from your library, Steam will prompt you to choose between the standard version and the uncensored UNRATED version. While often listed on various sites under "free
This edition includes all technical updates from the standard game plus additional content that was too extreme for the initial release: Agony UNRATED Free Download (Update 5)