You can set default reading options for all of your titles from the SETTINGS page in your publisher account, and you can also make changes to individual title records using the instructions below.
First, go to the TITLE DETAILS page for your title. In the SETTINGS section on the left-hand side of the screen, click on the orange arrow next to Reading Options (it is the second one down).
You can now select which type of file members will download. Click Save to update your title's reading options. Read more about each option here.