Open the report for editing, and navigate to the report section you'd like to change. 

Click on the page's heading in the report preview (to the right of your screen). You should see a rectangular outline around the heading you have selected, as seen below.

Next, type the heading you'd like to use for the page.

This heading will automatically apply in the Table of Contents, and in the navigation panel on the left side of the report.
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Alternatively, you can hover over the section, click on the ellipsis button "...", and click on Rename.

Then, type the heading you'd like to use for the page.