I saw in the Shoutbox that the War Spoils Beta is 28GB to download. This isn't surprising, as it would be a separate copy of the normal game with minor changes.
If your Internet connect is ADSL2+ quality or less (or has low data limits), you probably don't want to have to download 28GB from scratch. However, you should be able to skip most of that.
Locate your Game Directory
In Steam, find COH2
Right-click and select Properties
Go to Local Files
Select Browse Local Files
Copy Your Existing Directory
Make a copy of your current COH2 folder in Steam (in the same parent folder) and rename it Company of Heroes 2 - Test Build
Then when you initiate the Beta download, it should search for existing files and find most of them.
Note: This might only work if you haven't already started the download. However, if you have started, you can try copying your COH2 files and skipping straight to verification.
The most reliable way is to select the Delete Local Content option for the beta in Steam and start over. It's up to you whether that is worth it.
Verification
Once the download is complete, I recommend verifying the files to make sure the new ones replaced the old ones.
Right-click on the game in your Steam Library
Select Properties
Select Local Files
Click on Verify Integrity of Game Cache
That shouldn't take long, and you'll be good to go!
Please leave any feedback below.
Edit: Looks like Relic already suggested this themselves: https://community.companyofheroes.com/discussion/comment/244241#Comment_244241