@isomeone wrote:
How can I do an amend in Turbotax, for my 2018 return which was already e-filed but not through Turbotax ?
If you did not file your original 2018 return through TurboTax, you will essentially have to start an amended return from scratch in one of the versions (see the first link below with respect to amending in TurboTax).
Depending upon the provider you used, you might be able to transfer in some information from your original return.
Regardless, you will have to purchase a product (see the second link below).
TurboTax amending FAQ: https://ttlc.intuit.com/questions/1894381
TurboTax products: https://turbotax.intuit.com/