The most secure way to obtain the game is through official storefronts. Once purchased, Steam automatically keeps your game updated to the latest version (currently 1.20.x), but you can manually revert to older builds. : Planet Zoo on Steam Official Site : Planet Zoo Store How to Download Version 1.2.5.63260
Since this specific build is an older version, you must use the and SteamDB to fetch the original files.
: Look up the Planet Zoo (App ID 703080) on SteamDB and find the Manifest ID associated with version 1.2.5.
: Press Win + R and enter steam://open/console .