Group tasks are edited in the same place they were created.
Go to Tasks and select the task in the list. The same form opens with its tabs.
Change what you need and select Save (green icon at the top). The red icon archives the task.
Figure 3. Editing a group task: Save and Archive at the top. The grant is locked.
Figure 4. Donors tab: the selected donors are the ones who will sign.
Figure 5. Products tab: Min required, Maximum, Add details and delete.