Adding additional logins to your account is useful because it provides alternative ways to access your account in case you lose your password. You can choose Google or Facebook network-wide and GitHub on Stack Overflow only. You can also create a new Stack Exchange OpenID by providing your email address and creating a password.
Visit your user profile page (if you can't remember how to log in, go here and enter your email) and click the "Edit profile" button at the top right of the page. From there, click the "Your logins" tab:
You should see a list of your logins. For quick access, this link should take you to the page.
To remove a login:
Click the "remove" link next to the login/s you wish to remove.
To add a login:
Click the "Add more logins..." button below your current logins. Choose the new login from the list and follow the steps to confirm.
If your issue was not covered above, please feel free to contact us directly.