Students can change a group by going in and deleting their submission and uploading it again. This will allow them to correct the group members.
If the deadline has passed you'll need to allow late submission for the students who specified the wrong group.
- Go to the assignment overview by clicking on the name of the assignment in the sidebar.
- Go to the Late Submissions tab.
- Find the relevant students for the group submission that has the wrong group
- Click Allow
Now the students are able to hand in again with the right group.