You can only add 2018 deductions on a 2018 tax return. Since the 2018 tax return was filed you will have to amend the return to change, add or delete 2018 tax data from the original return.
See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579
Will you have enough itemized deductions to exceed your 2018 standard deduction? If not, amending will make no difference.
Many taxpayers were surpirsed when they prepared 2018 tax returns because their itemized deductions are not having the same effect as they did on past tax returns. The new higher standard deduction and the elimination of certain deductions, as well as the cap on state and local taxes have had a major impact since the new tax laws went into effect beginning with 2018 returns.
Your itemized deductions have to be more than your standard deduction before you will see a change in your tax owed or tax refund. The deductions you enter do not necessarily count “dollar for dollar;” many of them are subject to meeting tough thresholds—medical expenses, for example, must meet a threshold that is pretty hard to reach. The software program uses all the IRS rules that apply to the expenses you enter, and it tells you if you have enough to use your itemized deductions or if using the standard deduction is more advantageous for you. Under the new tax laws, some deductions have been capped—there is a $10,000 limit to the itemized deductions for state, local, property and sales taxes.
Your standard deduction lowers your taxable income. It is not a refund.
2018 Standard Deductions:
Single $12,000 (+ $1600 65 or older)
Married Filing Separately $12,000 (+ $1300 65 or older)
Married Filing Jointly $24,000 (+ $1300 each spouse 65 or older)
Head of Household $18,000 (+ $1600 65 or older)
Look at line 8 of your Form 1040 to see your standard or itemized deductions.