World Archives Project: What happens if I exit a World Archives Project image set before completing it?
From Ancestry.com Wiki
You can exit an image set at any time and the program will automatically save the information you have keyed.
If you decide that you cannot complete keying an image set, you can cancel it. Click the Select Image Set tab, then highlight the image set you wish to cancel, then click the Cancel Image Set button. This will clear out the data that you have keyed in and send the image set back for someone else to work on.