I already submitted my taxes but recieved another one in the mail and forgot to file it, how do i go about that?

Is the "another one" you received a W-2 or some other tax form that must be entered on your tax return? If the IRS rejects your return, you can make the changes, fix the reason for the rejection and resubmit it. If the IRS accepts your return, you should wait until you receive a refund (if you are getting one) and then you can prepare, print, sign, and mail an amended (changed) return form 1040X to make the changes. Form 1040X is estimated to be available in TurboTax on 2/22/19 but the date has changed at least 3 times. Do not change your original return until you begin the amended return. Do not spend your refund because you may owe it back with your amended return.