You can edit your activities at any time, as long as the task is still ongoing and is not marked as completed. In order to edit your activity, simply click on the "modify" icon next to it.
Please bear in mind that archived tasks cannot be edited, and only the network's administrator can unarchive a task. Therefore the activities on an archived task cannot be edited.
It is not possible to edit another person's activity, for security and legal reasons, since the activities are nominative, and therefore personal.
The network's administrator has only permission to delete the other members' activities, if necessary. He has not permission, however, to edit those activities.