If you missed a deduction on your 2016 tax return that applies to income earned in 2016 then you will need to file an amended 2016 tax return (Form 1040 X). The IRS information may be found here: https://www.irs.gov/newsroom/ten-facts-on-filing-an-amended-tax-return and the 1040 X instructions are found here: https://www.irs.gov/pub/irs-pdf/i1040x.pdf
You have 3 years in which to file the amendment.
Also see the following: https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend
Answers are correct to the best of my ability but do not constitute legal or tax advice.
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