By going straight to the download on the marketplace and it should say "download again" or your download history.
This.
Since anything you download is tied to your GamerTag, the Marketplace will show that the Beta Testers have already downloaded it. So after you delete Dawnguard off of your HDD, simply go to the LIVE Marketplace --> Skyrim --> Dawnguard -->instead of the option to "Purchase for 1600 msp" it will say "Download Again".
Choose that option and once it's done, you're back playing but with all of the fixes and tweaks put into place.