With some servers you can set the desired version in the Startup tab of the panel. (e.g. Minecraft or Terraria)
After that, all you have to do is click on Reinstall in the Settings tab. Don't worry, this won't delete any data (such as your world).
SteamCMD servers (e.g. Garry's Mod or Rust) always use the latest version and simply need to be restarted.