Sure, I can help you with that! Here are the steps to change the "Series" names in your chart to "John" and "Mary":

1. Click on the chart to select it.
2. Look for the "Chart Design" tab in the ribbon at the top of the screen and click on it.
3. In the "Chart Layouts" group, click on the "Add Chart Element" dropdown menu.
4. Select "Legend" and then choose "More Legend Options" at the bottom of the menu.
5. In the "Format Legend" dialog box that appears, click on the "Series" tab.
6. Click on the name of the first series (which should be "Series 1") and type in "John" to replace it.
7. Click on the name of the second series (which should be "Series 2") and type in "Mary" to replace it.
8. Click "OK" to close the dialog box and apply the changes.

That's it! Your chart should now display the names "John" and "Mary" in the legend instead of "Series 1" and "Series 2".