If your efiled return has been accepted you have to file an amended return. This FAQ explains how to file an amended return. 

https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...

If your efiled return is rejected you can just add the schedule c to it and then try to efile again.

Amend and add your Schedule C.

First, make sure that your 2020 return was accepted in TurboTax Online or paper-filed through the mail. You can't amend a return that hasn't been filed yet.

When you're ready to amend, find your situation below and follow the instructions.

TurboTax Online

TurboTax CD/Download

To set up your business:

1. Open or continue your return.
2. Search for schedule c and click the Jump to link in the search results.
3. Answer Yes to Did you have any self-employment income or expenses?
   • If you've already entered self-employment work and need to enter more, select Add another line of work.
4. Follow the onscreen instructions.