If your ex was listed/entered first on the 2019 tax return, then you can't just delete it and make you the first one listed. You have no choice but to start a new return (by creating a new account if using the online version) and starting over from scratch with your new filing status (single or MFS) with your 2020 tax return.
If using the CD version, then you can't import from the .tax2019 file either. Otherwise, all data entered for the first tax payer entered on the 2019 return, will be "tied to" the first tax payer on the 2020 tax return - and that would be you.
