I was over paid by my employer in 2016. I paid back the money in 2018. How do I fix my taxes to show less money earned in 2016 , due to paying back the over payment

To report the repayment of taxable income  -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Other Deductions and Credits
• On Other Deductible Expenses, click on the start or update button

Or enter other deductions in the Search box located in the upper right of the program screen.  Click on the Magnifying Glass.  Click on Jump to other deductions

If the repayment amount was $3,000 or less -

On the next screen, click on No (Generation Skipping Taxes)

On the next screen, click on Yes (Less Common Expenses)

On the next screen, enter the amount repaid. Continue with this section until it completes.

If the repayment amount was over $3,000 -

On the next screen, click on No (Generation Skipping Taxes)

On the next screen, click on No (Less Common Expenses)

On the next screen, click on Yes (Do You Have Any of These Other Expenses?)

Enter the amount repaid in the box labeled Claim of right repayment over $3,000. Continue with this section until it completes.

the deduction or credit goes on the 2018 return.  you can not change 2016.