I can't find the way to amend my 2016 tax return.

See this TurboTax support FAQ for amending a 2016 tax return originally completed using TurboTax (online or desktop) - https://ttlc.intuit.com/questions/3061514-how-do-i-amend-a-2016-return-in-turbotax

For the online editions - 

You have to sign onto your 2016 online account using the exact same user ID you used to create the account.

Close all TurboTax windows on your web browser (including this one).  Copy and paste the account recovery website link onto a new web browser window and run the tool.

Use this TurboTax account recovery website to get a list of all the user ID's for an email address.  Run the tool against any email addresses you may have used - https://myturbotax.intuit.com/account-recovery

If none of the user ID's received will access your 2016 account, then use the option shown in blue on the account recovery website, "Try something else"

To access your prior year tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Amend (change) 2016 return