How do I edit my Organization's booth design and change or remove the Avatar?

You can make any edits to your booth design and booth avatar under My Organization and then select Booths.

Under Settings on the left-hand side of your screen, select My Organization, then select the tab that says booths.  In this section you can edit/add custom links, change your booth design and avatar as well as viewing your completed booth just as a candidate will see it by selecting the View as Attendee Button.