If you need to add a UPN because the student currently doesn't have one, follow this article on Adding a UPN to a student.
The current UPN is incorrect
If the UPN on a Student Profile is incorrect, you can change this by removing the existing UPN, and then adding a new one.
Go to the Student Profile and click on UPN in the Identity section.
Click the orange Edit button.
Click the red Delete UPN button.
Click the red Delete button. You'll then need to add in the new UPN.
The UPN is Temporary and needs to be changed
If the UPN was Temporary in Arbor, it will say Temporary UPN under the Identity section of the Student Profile.
To change this to a permanent UPN, click on the Temporary UPN, then click the orange Edit button.
Untick the Temporary box
Enter the UPN in the Existing UPN field, or leave this blank for a random UPN to be assigned.
Click the green Assign UPN button.