When you to a chart in Excel 2013, Excel builds the legend using data from the spreadsheet. Labels in the legend match those in your header row, and the colors in the legend match the colors in the chart. The legend itself sits below or to the side of the chart, out of the way of the data. To change the legend's position, appearance or contents, format it, adjust the colors and change the series names.
Format the Position, Background and Text
Double-click the border of the legend to open the Format Legend panel. In the initial section, set the legend's position relative to the chart. The Show the Legend... check box prevents the legend from overlapping the chart.
Open the Fill & Line section (the paint bucket icon) for options concerning the legend's background color and border.
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Use the font settings on the Home tab to change the font or text size, just as you would edit regular text. For advanced style options, such as text fills or outlines, click Text Options on the Format Legend sidebar.
Edit Legend Colors
Click the legend, then double-click a particular entry on the legend to open the Legend Entry Options sidebar. Set the border color in the Fill & Border section to change the color for both the legend entry and the corresponding line, pie slice or bar in the chart.
Edit Legend Text
Legends usually draw their text from the spreadsheet. Change the header on a column and the legend updates automatically. To edit the legend's text without changing your spreadsheet, change the series names instead:
Click your chart, and then click Select Data on the Design tab
Pick the entry you want to change in the Legend Entries (Series) section and click Edit.
Type a new name in the Series Name box, and click OK on both windows.