At the Content tab, select 'Agenda' on the event app menu, followed by the 'Edit' icon of the selected session.
To reorder the Session Groups, select the Groups tab. At the selected Session Group, click the '＝' icon and drag the group to the desired location.
The session group order will be updated automatically after you have reordered the groups.
Speakers in the session groups may be reordered as well. Please refer here for steps on reordering speakers in the session groups.