Once an entry is submitted to contests, it cannot be changed. However, it can be removed and resubmitted. The reason why it cannot be changed is because since a contest holder may have already reviewed that entry, we do not want to change an entry that they’ve already seen and possibly even rated.
You can always submit minor variations to an entry as an additional entry. If the entry does not have a high rating, it may be a better option to remove or withdraw that particular entry that you are replacing.