Head to Reporting > Reports > Students (Blue Tab) and choose Custom Report, then tick the UPN box before choosing. This will bring a report up for you with 3 columns. Under the grey UPN column there is an empty white box - in here type in the UPN number you have and enter. This will bring the pupil up you are looking for. If this is a former student you seek, ensure you have ticked to include Former Students prior to running the report.