You can move students within groups. However, changes should be made before assigning a group to an assignment.
If a student is added or removed from a group after receiving submissions, the submissions and grades may not align correctly. If a student is added to a group after the group submission, any re-submissions will overwrite the original submission; all previous grades will remain but the Gradebook will show the assignment was resubmitted since last graded. If you want to move a group member with a graded submission, you may want to clone the group set instead.
Open Group Set
Click the Settings icon , then click the View User Groups link . Or click an existing group set .
Move Student via Drag and Drop
Expand the group and find the student you want to move. Click the student's name and drag it to another group.
Move Student via User Settings
You can also click the student name Settings icon  and select the Move To... link .