You;ll have to amend your 2016 tax return to add the donations.
Please note that donations are an itemized deduction on Schedule A. If you use itemized deductions in 2016, then it is worthwhile amending as these donations would increase your itemized deductions.
If you used the standard deduction in 2016, then check the omitted donations when added to the other possible itemized deductions would exceed the standard deduction. If yes, then go ahead and amend. If no, then there is no tax benefit in amending.
To amend your 2016 tax return, please follow this link:
https://turbotax.intuit.com/personal-taxes/cd-download/amend/ty16/
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