1. Open the Properties dialog.
2. Select the 'Symbology' tab and the 'Classification' 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.