1. Make sure the game and any servers aren't running.
   
2. Open your Steam library.
   
3. Right click on 'A tractor' in the game list.
   
4. Select 'Properties'
   
5. Select the 'Local Files' tab on the left.
   
6. Select 'Verify Integrity of game files..'
   

This will force Steam to check for any update files and make sure all the files you have locally are up-to-date.

To check your current game version, use .vers to print the client version and *getvers to show the server version.