To search for a student by their UPN, you can go to Reporting > Custom Report, New report
Click on Run Report, then enter the UPN in the search box to filter the report.
Alternatively you can go to Reporting > Reports > Students (tab) > Custom Report
Select UPN, and possibly year Group and Form Group, then click Choose to run the report.
Enter the UPN in the search box for UPN, to filter the report
If you click on the student's surname, it will take you straight to the student profile.
If you are uncertain if the student is no longer on roll, you can run either report by selecting Current and Former (Off Roll) Students to include all students in the report.
If you know the student is off roll, go to Admin > Students > Search Not On Roll. Limit results to All Students then click Choose to run the report. Here you can filter the UPN column in the same way by entering the student's UPN number.