Amend 2019 Return via Turbo Tax already filed by Accounting Firm

I had an accounting firm file my 2019 Taxes, which I've already received my "paltry" 2019 Refund. Tax return was MFS (married filing separately), with 1 child, even though we had 2 children. :(  STANDARD Deduction was taken.

My spouse, will file her 2019 Taxes soon (MFS with 0 Dependents), but since she has her own "side" business, in addition to her full-time position, she will need to "ITEMIZE" her deductions. OK so far?

I want to AMEND my 2019 taxes THROUGH TuboTax, changing from STANDARD to ITEMIZED deductions, and claiming the 2 children as deductions, in addition to GLARING errors by the accounting firm. (I am FINISHED with this firm for good!) I just did my taxes through TT using the same numbers I gave this firm, and instead of the "paltry" few hundred dollars refunded, I will get in excess over $3k.

So...I've already entered this information through TT, how can I change it to the 1040X, from the 1040 SR? Or do I have to start over?

Thanks in advance for any help! I appreciate it!

 

JR