What are the issues you are currently facing?
How about trying this: Try to restart Steam, and see whether it updates when you have restarted it.
If it doesnt, maybe try to do the following:
1. Right click on Rebellion Beta
2. In the Properties dialog box, go to Local Files
3. Then, click the Verify Integrity of Game Cache.
4. It should cross check your current copy with the file on Steam server. If there are any difference between the files, Steam should auto update for you.
Please post update whether this works or not.