First, you must be logged into your books as the Admin since only the book’s Admin can change user privileges. If you do not see Admin in the top-level toolbar, shown in the screenshot below, then you are not logged in as the Admin.
Once you are logged in as the Admin, click on Admin in the top-level toolbar, shown in the above screenshot. You will be brought immediately to the User/Group Permissions tab where privileges may be changed.
To start, click on the arrow to expand the group that the user is in. If you do not know which group the user is in, begin by looking in the Unassigned group, as that is where users are automatically placed if they are not assigned to a group.
Click on Edit next to the user for whom you wish to change privileges.
This will open the user permission screen. Check or uncheck the privileges you wish to add or remove from the user and then press Save when you are done.