You enter any form dated 2018 on your 2018 tax return. 2018 forms for 2018 distributions are issued in early 2019. 2019 forms for 2019 distributions will not be issued until 2020.
If you need to amend a 2017 tax return, you can't use the current online product to do that. You must use 2017 software.
See this FAQ for instructions:
https://ttlc.intuit.com/questions/4252298-how-do-i-amend-a-2017-return-in-turbotax
