The IRS will not accept (or reject) returns until Jan 28.
If you filed, it is like dropping it into the mail - you cannot get it back if accepted by the IRS or state.
To amend your 2018 tax return:
-- First, *wait* to see if your return has been accepted or rejected by the IRS or state. DO NOT do anything until you receive the accept or reject e-mail.
-- If rejected, you can correct and re-send your return.
-- If accepted you should *wait* until your return has been processed and you receive your refund or conformation that any tax due has been paid. (If you file an amended return while you first return is being processed it can cause extended delays for both returns if two returns are in the system at the same time). In addition, if the IRS makes any change on your original return, you might end up having to amend the amendment – a sticky process that can take a year or more).
-- Then you can start the amend process.
-- Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing.
See this TurboTax FAQ for help with amending:
You can check the status of your amended return here but allow 3 weeks after filing for it to show up: