I don't see the "change" or "amend" return when I hit 2018.

Are you trying to amend an already accepted 2018 return or a return that has not yet been e-filed?

It has already been accepted for 2018

To amend your return, you can follow the instructions here: 

https://ttlc.intuit.com/replies/3288565

Specifically for TurboTax Online: 

https://ttlc.intuit.com/replies/7276596