If the amount is more than $3000, as in your case you have more to do than just where to put the amount. This is from pub 915:

Repayment of benefits received in an earlier year.

If the total amount shown in box 5 of all of your Forms SSA-1099 and RRB-1099 is a negative figure, you may be able to deduct part of this negative figure that represents benefits you included in gross income in an earlier year, if the figure is more than $3,000. If the figure is $3,000 or less, it is a miscellaneous itemized deduction and can no longer be deducted.

You need to research pub 915 to see the steps you need to take.