When/how can I amend my 2019 Fed return online through TT that has already been accepted?

If the tax return is accepted then you will have to amend the tax return to make your changes. An amended tax return, Form 1040X, can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return. Wait until you have received the tax refund or the taxes owed have been paid and processed by the IRS before amending the tax return.

On the Tax Home web page when the Form 1040X for amending a 2019 tax return becomes available there will be a link - Amend (change) return

Form 1040X is estimated to be available on 02/27/2020