To amend a 2018 tax return using the online editions, see this TurboTax support FAQ - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577
To report capital gains or losses you will need to upgrade and pay for the Premier online edition or higher. Click on Upgrade on the left side of the online program screen when accessing the 2018 online tax return.
