You can only edit Reports you have created or have permission to edit. If you wish to edit a different Report, you will need to make a copy of it first (see copy a Report if you're unsure how to do this).
From the Reports homepage, click on the Edit button next to the Report.
Alternatively, if you have already run the Report, click on the Edit Report link at the top of the screen.
You will be taken to the Report editing screen and can click onto the tabs to edit the different parameters of the Report.
Please see the create a new Report guide if you need help with the options available in each tab.