In TurboTax Online, under My Info, select I am preparing this return for xx who has passed away.
You may prepare an IRS form 1310 Statement of Person Claiming Refund Due a Deceased Taxpayer. The form is used to claim a refund on behalf of a deceased taxpayer.
In TurboTax Online, enter 'form 1310' in the Search magnifying glass in the upper right hand corner of the screen. Select 'Jump to form 1310'.
See also this TurboTax Help.