Hi,
You can change it if you export the performance report to XML, change <UNIT>YOUR_UNIT</UNIT> and <UNIT_SHORT>sh</UNIT_SHORT> and then import it back. You can find export/import functions in open/save dialog in the performance report designer portlet.
unit is the one that displays on the axis and unit_short in the legend.
the XMLs are also available in your database in table umpQoSReports.
-jon