To edit a chart legend:
Double-click on the chart.
A grey border appears around the chart and the menu bar now contains commands for editing the objects in the chart.
Choose Format - Legend or double-click on the legend. This opens the Legend dialogue.
Choose from the available tabs to make modifications, then click OK.
To select the legend, first double-click on the chart (see step 1), then click on the legend. You can now move the legend within the chart using the mouse.