Did you successfully NETFILE your 2018 tax return using TurboTax? You’ll only see the REFILE button next to your return if you sent your return by NETFILE and you got a confirmation number from the CRA.
To do a ReFile to your 2018 return, you'll need to sign in to your 2018 version of TurboTax Online and click the ReFile button next to that return and make the changes. (If you used the CD/Download version, you will need to sign in to that year and select "change a filed return".
This FAQ has more information on making changes to a return that has already been filed: https://turbotax.community.intuit.ca/community/tax-return-amendment/help/i-have-filed-my-tax-return-...