Andy,
If your Dynex is the same as mine, you should see a "MENU" button on the remote. This will bring up various settings. Push the right arrow button until you get to the channel menu. The first menu item lets you pick between air and cable. The second menu item lets you perform an auto scan.
I found the user manual online here:
http://www.google.com/search?sourcei...2_ENG+Final+lr