Select your OS and language to see if 0.25 is still hosted in the archive list. Kerbal Space Program Forums Third-Party Archives (Use Caution)
is a paid commercial release and is generally not hosted legally on third-party sites. System Requirements for 0.25
If you purchased the game directly from the Kerbal Space Program Store, you may be able to find older installers in your download history, though version 0.25 is rarely kept on active servers.