There is no undoing the consequence of the rollover being to a Roth IRA. If you did not report the rollover from the 401(k) to the Roth IRA as taxable on your 2016 tax return you must amend your 2016 tax return to do so unless the IRS has already caught the error and billed you for the taxes due.
If not already reported as taxable on your 2016 tax return, presumably the 401(k) plan issued a code G Form 1099-R with $0 in box 2a, thinking that the rollover was to a traditional IRA. When using 2016 TurboTax to amend your 2016 tax return you'll need to change the amount in box 2a to be the same as the amount in box 1 if you had no basis in after-tax money in the 401(k) and provide explanation that the rollover was a taxable rollover to a Roth IRA, not a nontaxable rollover to a traditional IRA, despite what was reported in the 2016 Form 1099-R. (If the code in box 7 of the 2016 Form 1099-R was code 1, 2 or 7, you'll just need to change your answer to 2016 TurboTax's question asking what you did with the money.)
