To manage/edit Students' Roll Number in USM, you will have to go to Student Module, which is accessible through the Menu Bar on the top right of the screen.
Step 1. Click on the Student Module under the Menu Bar.
Step 2. Click on Manage Roll Number on the left sidebar of the screen. 
Step 3. Select the Academic Session for which you wish to edit the Roll Numbers. 
Step 4. Click on either of the two options next to Sorted based on to change the criteria through which the Roll Numbers get assigned. 
Step 5. Click on Set Roll No Info. next to the class in which you wish to edit the Roll Numbers. 
Step 6. If you wish to change the Roll Number Prefix or Postifx, you can change them in the given dialog box. Click on Assign after editing the Prefix or Postfix. 
Step 7. Click on Assign Roll Numbers next to the class in which you wish to edit the Roll Numbers. 
Step 8. Click on Reset All option at the end of the list. 
Step 9. Click on Assign after once having gone through all the Roll numbers and confirmed that the new Roll Numbers are in order.