Click on Upgrade on the left side of the TurboTax online program screen when accessing the 2019 online tax return.

TurboTax has 2 free products.

If you're using Free Edition at the regular Online TurboTax website, you can upgrade as DoninGA provided above.

If you are referring to the TurboTax IRS Free File program at its special website, then the method to go from that free TurboTax IRS product to a paid commercial product edition is different.