Hello there, rekindled-life-y.
You can only associate a single QuickBooks Self-Employed account to your email address. Hence, switching from one QuickBooks Self-Employed account to another is not possible. You'll need to log out and log in using the other QBSE credentials.
Note that the Switch Company option is only available in QuickBooks Online.
Let us know if you need anything else. Have a great day!