A 1099 DIV with an amount in box 5 is entered into TurboTax like a regular 1099 DIV form.
1099 DIV forms with amounts in box 5 are allowed a 20% Section 199A deduction (also known as the Qualified Business Income deduction) of the box 5 amount from their taxable income (unless you also have other qualified business income and then the calculation gets more involved). Therefore, if the dividend income was already reported on your return, an amended return (form 1040X) may give you a small refund back as your taxable income would decrease by the 20% deduction.
See instruction from the IRS below:
Generally, you are allowed a deduction
up to 20% of your net qualified business
income plus 20% of qualified real estate
investment trust (REIT) dividends and
publicly traded partnership (PTP) income.
Qualified REIT dividends include any
dividend you receive from a real estate
investment trust held for more than 45
days and for which the payment is not
obligated to someone else and that is not
a capital gain dividend or qualified dividend plus your qualified REIT dividends
received from a regulated investment
company. This amount is reported to
you on Form 1099-DIV, line 5.
From: https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
Plus, below is more information on how to amend a 2018 tax return in TurboTax. Note that amended returns must be printed and mailed, as they cannot be e-filed.
