Sometimes the installer "thinks" it is up to date because of corrupted temporary files.
Temporarily disable your antivirus or add the game folder to the Exclusions list. Summary Table: Common Fixes Recommended Action Old Beta Version Uninstall and download the "Live" version from the store. Corrupted Files Use "Verify Integrity" in your launcher. Write Access Run the installer as Administrator. Legacy Installer
This forces Steam to compare your local files with the current server version and download any missing or outdated components.