Permission required: End-user support
Unfortunately, once a user downloads a title in a specific format, they can't download it in another format.
Instead, your user has two options: they can read the ebook in their browser using OverDrive Read, or they can borrow the book again and choose a different download format. (If they're using the OverDrive app, they may need to switch to their device's web browser to see the read-in-browser option.)
For the second option, you can return the title from the user's account using the Return titles feature (under the Support tab). Once returned, the user can either borrow the title again (if it's available) and download it in a different format, or place a hold on it (if it's unavailable). If the user needs to place a hold, you can move them to the front of the holds list using the Manage holds feature (also under the Support tab).