Since you have already filed your tax return, there is no need for an extension request.
You will have to amend the tax return that was filed and accepted by the IRS. An amended tax return can only be printed and mailed, it cannot be e-filed. You can amend your tax return at any time. If the amended return indicates that there will be taxes owed, then you there will be a late payment penalty for those taxes owed. The IRS will send you a bill in the mail for any penalties. If the amended return indicates that there is a tax refund, then the IRS will send you the refund as a check in the mail.
See this TurboTax support FAQ for amending a 2016 tax return - https://ttlc.intuit.com/questions/3061514-how-do-i-amend-a-2016-return-in-turbotax
