In the "Manage Global Permissions" window, a list of users for which you have permissions defined is displayed. Each user's permissions can be deleted or modified by selecting it and using the "Delete" or "Edit" buttons.
If you select "Edit", then the "Manage permissions for user" window will appear and you can modify the existing settings according to your requirements.
In the "Effective permissions" tab you will be able to see what permissions are specifically allowed or not.
Effective permissions are displayed only for already saved permissions, so changes won't be reflected until saving.
The "Effective Permissions" tab will only display the relevant permissions after you select a user/group and it will only show updated permissions after you save your settings.