Once a staff user account has been created, you may need to edit their information or user permissions at some point down the road. To do so, start under Users > All Users and click on the name of the user that you need to modify.
When viewing a user account, you can modify their user information (other than their username). You will also be able to add new permissions to a user account by checking the boxes next to each permission. When you have made all desired changes, be sure to hit the Save button at the bottom.