Step 1
You can update the login credentials to your linked accounts using the "Manage Linked Accounts" menu item located in the dropdown menu of the the "Link Accounts" menu as show here:
Step 2
In the screen that opens, select the "Update Credentials" menu item in the "Refresh Site" dropdown for the site you wish to update as show here:
Step 3
You can now enter your updated credentials as show here:
