Please see the TurboTax Help article How to amend (change or correct) a return you already filed for instruction on how to amend your previously-filed tax return.

You should wait to file your amended return until you get any refund you may have coming from your original return.  Even if your original return has been accepted by the IRS, it's still possible that the IRS could change your refund or balance due on that return. 

My 2020 return has been accepted but the box is checked that says someone can claim your spouse as a dependent which is incorrect. I don’t know why that happened. I’m married filing separately which is the same as last year but this didn’t happen. What should I do? Thank you. 

If you are married filing separately, and your spouse has already filed, you may get a message that another return has been filed with your spouse's SSN.   It's odd that you would get the message that he's been included as a dependent on someone else's return.  Double check your e-file status using the steps in the following TurboTax Help article:  How do I check my e-file status? - TurboTax  

Once you receive confirmation that your federal return has been accepted by the IRS, there is nothing you need to do.  You'll be able to start tracking your refund at the IRS Where's My Refund? site.

@Amyleasm