Choose the branch from the dropdown menu. Steam will download the legacy files.
: To go back to the current game (3.0+), go back to the Betas tab and change the selection to Key Differences in 2.2.5 unturned 2.2.5 download
: Official servers for 2.2.5 are no longer active. You will likely only be able to play Singleplayer unless you host a private server using the older server files. Choose the branch from the dropdown menu