1. Open the Properties dialog.
2. Select the 'Symbology' tab and the 'Point Source' tab within it.
3. Within the Label column, click twice over the label you wish to edit. After a slight pause, a text box should appear so that the label can be edited.
4. Input a new label into the text box.
5. Press the "Enter" key on the keyboard or click outside the text box to accept the new label and complete the edit.