As an Admin, you can update several items about the account for a family.
First, search for the student within the Applicant Index. Next, click to view their student profile.
You will see an account settings ribbon at the top of the profile.
You will be able to edit the account's username by clicking the edit button next to Account Login:
Then, type in the family's desired username and click Change:
If the username is already in use by another account, you will see the following error message:
If an account has more than one student, you only need to update the account information once. All students profiles linked to the family account will also update.