As long as there are no restrictions in place, the easiest way to do this would be from the list view. You would have to do the Assignment Group and the Assigned To separately but you can update multiple records at a time.
This link provides further details:
https://docs.servicenow.com/bundle/kingston-platform-user-interface/page/use/using-lists/task/t_EditMultRecUsingListEditor.html
