When changing the virtual name only the visible subject name is changed, the subject's file is not really changed. Virtual subject names are a good way of including punctuation in a report.
Select the subject you want to rename, then right-click, a menu will appear. Click 'Set virtual name'.
You can enter a new subject name here. Unlike normal subject names you can also include punctuation characters.
Once the name is set, click OK to return to options, notice that the new name is in the virtual name column.
There is a useful trick to customise subject titles per pupil using virtual names.