Instructions
Change the dashboard
Turn on the XBox and take your disk out of the tray. Most modded XBox's are set up to auto-run any disk that you put into the tray. You can keep them from doing this by removing the disk, or leaving the tray open.
Navigate to the "System" option of your home screen. Press "a" to move to the next page.
Click "Skins" from the four options, and click "A" to move to the next page.
Select the skin that you want from this screen. Depending on how many you installed when you did the softmod, you may have as many as several thousand options here. Scroll through and find the skins that you'd like, and click "A" to set that as your dashboard.