An external login may be modified by going to Settings > Accounts and Security > External Logins, clicking on Configure, and then clicking on Select next to the desired user.
Then scroll down to the Login Section and click on Edit login/profile.
By default, the system removes the current password when a login is edited.
You will need to enter and retype a password as well as make any additional needed changes to the external login. Then click on OK.
The Login Section will appear as shown below after the changes are saved.