Yes, Stu is exactly right! You can drag and drop an existing cue from the script area on the left side of MediaShout and onto the Quick Cue that you want to replace. Or, you can right-click the Quick Cue and choose Edit and then use Change BG to change the background and make any other changes necessary. Then, click Save and Return to Script (beneath the editing area).
Let me know if you have any trouble with this!