To Edit a User Login, you will need to go to the Account the user is in. Scroll Down to the Users section, then click on Edit
You may change whatever info you need to change for the user.
If you wish to restrict the user to a particular Site or have him see only the site he has under that account, click on the "Restrict to Specific Sites" button
Once there, you may select a single site
Or multiple sites

Hold the Ctrl Button on your Keyboard and select a Site to select Multiple Sites
If you wish for your User to only be able to see the dashboard and not make changes to it, you may set their Red Only Restiction to Restricted
You may also Disable or re-Enable a User's Access whenever you want