First, wait a bit to see if your e-filed return was rejected or not (you did e-file it, yes?)
If the return is rejected by the IRS, then re-open the return, fix the problem that the reject code points to, fix the other thing that is wrong, and then e-file it again as if it is for the first time.
If your return has been accepted by the IRS, then wait until you get your refund (if one, otherwise 3 weeks or so), and file your amended return. No, the amended return does not have to be filed by April 15th.
Please see this TurboTax FAQ for how to amend your return with TurboTax - https://ttlc.intuit.com/replies/3288565
