Reporting Scopes work in conjunction with the System Roles. For more information, refer to this guide.
Locate the User you wish to amend and click on their name (see how do I find a User if you're unsure how to do this).
Click on the Set user roles link in the Common Tasks area (you can also click on the System Roles tab on the left-hand side if you prefer). 
Select the required option from Reporting Scope dropdown menu.
Click on the Save Changes button underneath the Reporting Scope options.