There are several ways to take on changing the text in the legend of a chart. You can modify the legend text for one series at a time, or you can edit the default legend to change all series' legend text at one time.
Edit one graph's legend
Double-click on the graph for which you'd like to modify the legend text. Alternatively, you can click on the graph once and find the "Edit" button next to graph text under the Presentation properties tab.
In the edit window, you can type the custom label of your choice. You can also use the dynamic text option available on the right side.
Edit the default legend text
To edit all the series' legends at once, click on the legend. Under the Presentation properties tab, click the Edit button next to "Default legend text" in the Content group.
This is generally used together with dynamic properties, for instance if you want only to display the country of each series.