I amended my taxes last year, now i owe refunds I've never received and i owe my first tax refunds before i changed it.
If you amended your 2015 tax return and now have to pay back some of the 2015 refund you received, you either have to write a check and send it to the IRS, or pay online, clearly identifying your payment as being for tax year 2015, at https://www.irs.gov/payments/direct-pay