You can assign UPNs to students in bulk at Admin > Students > Bulk UPNs
In this area you can see which students on your system do not currently have a UPN assigned to them. Select the students you want to generate a UPN for (or Check All to select them all). Click Submit and the UPN will then pull through to show on the student's profile.
A student's UPN is found on their profile's extended tab under School Information, as well as on the left side of the front page.