First, you will want to wait until your tax return is accepted or rejected. If it is rejected, then you can go in and fix the mistake. If accepted, then you will want to wait until your original tax return is processed by the IRS. If you are receiving a refund, you should wait until the check is issued. Amended returns must be filed by mail.
To amend a 2017 return, follow the steps in the article below (The 2017 amended tax return instructions will be available sometime in late January/Early February):