All members can modify an entry - including changing the photo, title, date and description - for up to 5 days after it has been published.
To do this, go on to the entry and click on the 'More Info' button beneath the tags section.
Click the pencil icon button that appears and follow the instructions.
After the 5 day cut-off basic members can only modify the entry's tag and location. For paying members there is no time restriction on modifying entries, it's one of the benefits of paid membership.