I'm sorry, but you won't be able to amend an accepted 2018 for some time. Once the return has been accepted, it must be completely processed as is by the IRS. Once you've made payment or received your refund, you can then amend the return.
TurboTax won't likely even offer the amend feature until mid-February at the earliest. I'll share the link below for amending the return using TurboTax once the option is available.
https://ttlc.intuit.com/replies/3288565
Note: Amended returns cannot be e-filed. They must be printed and mailed.
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
