When a contest/challenge is open for submission, you can go back to the contest/challenge page and edit your entry.
Once you have entered a photo to a contest/challenge, the "Submit Photos" button will change to "EDIT ENTRIES"; simply click on the "EDIT ENTRIES" button and on the Edit Entries window, click on the small photo thumbnail at the TOP LEFT CORNER where it says 'Click the photo(s) you’d like to remove.' to remove the photo (from the contest/challenge) then select a thumbnail from the bottom that you'd like to enter instead. Make sure to click on 'Continue' once you are done editing your entries.
