Yes you need to amend to add Schedule C to your return. You can keep the original refund. The Amended return will only be for the difference you owe back. If you have a Net Profit from your business on Schedule C you will owe Self Employment tax on it. And by increasing your income you may lose some credits you qualified for.
How to amend a 2020 return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/01/27439
What version did you use? You may have to upgrade. You can enter Self Employment Income into Online Deluxe or Premier but if you have any expenses you will have to upgrade to the Self Employed version. Or any of the Desktop programs. But you will get the most help in the Home & Business version.
How to enter income from Self Employment
https://ttlc.intuit.com/community/self-employed/help/how-do-i-report-income-from-self-employment/00/...
