Instructions
Turn on your Xbox 360 and sign into Xbox Live.
Press the green "X" button on your controller.
Scroll to the "Settings" tab and press "Account Management."
Select "Download History." Here, you'll see a multipage history of every item you've ever downloaded onto your Xbox 360.
Select the item that you want to download in this list. This will bring up a secondary window with additional information about the item. Press "Download Again" to begin rdownloading the item.