This guide will help you on how to Steam update your server.
- Stop your server
- Press the Steam Update button on your control panel
- Will open a black window with the update process, wait until it finishes
- Start your server
You can force steam update validation check by clearing the steam cache, just delete the \steamapps\ folder from the root of your server to clear the steam cache before update.
It's recommended you do a backup of your server when possible before doing the steam update.