You can move students within groups. However, changes should be made before assigning a group to an assignment.
If a student is added to 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.
Note: In group assignments, all students should be assigned to groups before the assignment is published. Grades and submissions may be affected for students who were not in a group at the time of assignment submission but who were later added to a group.
Move Student via Drag and Drop
Expand the group where you want to move the student . Click the student's name and drag it to another group .
Move Student via Move To Option
You can also click the student name Options menu  and select the Move To... link .