Tax Year Prior to 2020: We already filed our taxes and got them back not knowing we had to file our schedule c with it what do we do now?

Amend

See this FAQ

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577

Now that your return has been accepted and processed, you will need to amend your return using Form 1040-X.

Form 1040X can't be e-filed per IRS regulations.

After you finish amending in TurboTax, we'll guide you through printing and mailing your 1040X along with any required attachments.

\Three weeks after mailing your amendment, you can start tracking it at Where's My Amended Return? on IRS.gov.