To edit the group members on a submission, you'll have to change the submission. If the deadline is open you will be able to click 'change submission' and edit the group members.
If the deadline has passed, you will need to contact a teacher.
To change the group members for a submission that has already been submitted you need to allow the students to submit again and then have them submit with the right group members.
The way you allow students to resubmit is different for a Live Session and a Homework Session:
If your assignment is a Live Session, then follow this guide: http://help.peergrade.io/how-to-s/assignments/how-to-allow-a-student-to-re-submit-in-a-live-session
If your assignment is a Homework Session, then follow this guide: http://help.peergrade.io/how-to-s/assignments/how-to-allow-students-to-submit-work-late
Allowing students to resubmit will not delete their work or the feedback given.