To amend a tax return for tax year 2018 where the original tax return was completed and filed using the online editions, there is no software to download. You amend the tax return using the 2018 online edition after the tax refund has been received or the taxes owed have been paid and processed.
See this TurboTax support FAQ for amending a 2018 tax return - https://ttlc.intuit.com/questions/4667233-how-do-i-amend-a-2018-return-in-turbotax
To amend a tax return for prior tax years (2015 thru 2017) where the tax return was originally completed and filed using TurboTax see this TurboTax support FAQ - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed