First, use the "Make Changes" button to bring up the edit screen. This is only available to group organizers.
Second, click anywhere inside the box below, and select a new deadline from the calendar that appears. Then be sure to click "Save changes" at the top of the page to exit Edit mode and update your Pay By Group. Your group members will automatically be notified that the deadline for the group has changed, but you should usually leave a comment to explain what's happened.
TIP: You can edit the deadlines even once the group has expired, or passed its deadline. Just move the deadline to some point in the future to re-open the group!