Add a chart title
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In the chart, select the "Chart Title" box and type in a title.
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Select the plus sign.
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Select the arrow next to Chart Title.
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Select Centered Overlay to lay the title over the chart, or More Options for additional choices.
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Right-click the chart title to format it. with options like Fill or Outline.
Update a chart title
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Select a chart title.
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Add an equal sign to the formula bar.
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Select the cell you want to link any title changes to.
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Press Enter.
Source: Microsoft