How to modify an existing user account
On the sidebar, click on Users tab.
Hover over the user account in question, until the four "modify" icons appear.
Going from left to right, those icons enable you to:
Edit the user account. After clicking this button, you'll see options to edit the user's profile, campaign access, or permissions as described here.
Login as the user. This option allows you see the dashboard exactly as the user will see it, and allows you to test the permission settings that you've applied for them.
Delete the user's account.