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, press the enter/return key to filter the report.
Alternatively you can go to Reporting > Reports > Students (tab) > Custom Report
Select UPN, then click Choose to run the report.
Click on Run Report, then enter the UPN in the search box, press the enter/return key 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 in the search box, press the enter/return key to filter the report.