There is an option to ‘Enable Advanced Reporting Mode’ under Reports > Export Settings > Advanced.

Tick the box and click Update to turn on this setting.

Enabling this option displays an extra button to adjust the Grouping of an existing report. 

For more information on using the report adjustment options, see the section on ‘Update Report Selection/Filters’.

For more information on how result grouping works, see the section on Step 4 and 5 in 'Create a New Report'.