The sign-in page defaults to the wrong Google Account username, and I can't change it.

In the Sign in with your Google Account box, click Sign in as a different user.

If you use the same computer to sign in to both your business and personal Google Accounts, always make sure that you're signing in with the correct username for the account. For example, if you sign in to your personal Google Account first and don't sign out, the username associated with your personal Google Account will be automatically filled in when you try to sign in to your business Google Account. When this happens, click Sign in as a different user and sign in with the username associated with your business Google Account.

Still can't sign in? Try this Google Accounts Troubleshooter.