Actually I have this exact question. Kind of. I filed my mother's 2019 return (as I usually do, as her) in 2020 , 2 months after she died. The return was accepted but the refund from that year has still not appeared.
I opened the TurboTax 2019 return to try to amend it (to get it to add Form 1310), but there doesn't seem to be a way for me to indicate she is deceased (in the amended process). Is there a way I can manually add a Form 1310?
You do not need to prepare an amended tax return to prepare Form 1310. This form can be mailed into the IRS separately.
Please see the attached link for the blank form and instructions. Assuming you are filing as
Blank Form 1310 - Statement of Person Claiming a Refund Due a Deceased Taxpayer
You will need to mail this form to the IRS office where you file your mom's return.
Please see the attached link with instructions on where to file your return. The addresses vary by state.