The 1040X form is not available yet in TurboTax. Currently, it is estimated that the form will be available on February 22.
When the form is available, the link below will be updated with instructions for how to file a 2018 amended tax return (1040X) which you can then use to help you prepare your amended return. Note that amended returns must be printed and mailed, as they cannot be e-filed.
https://ttlc.intuit.com/replies/3288565