Sorry no, you cannot file or amend a 2018 return with the 2019 Online version. The following will give you instructions on how to amend a 2018 return.
How do I amend a 2018 return in TurboTax?
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579
