Yes - you can edit and re-run an activity after it has been completed.
- Log onto your teacher account
- Select Reports (on the top of the page)
- Select View work (by default Charts would be selected)
- Click View all in the Work results box
- Click 'edit' next to the activity you would like to set
- You can then edit the activity before setting it again for your student
If you are using the old teacher dashboard click Activities (panel on the left) and select View activities. Follow step 5 on wards.