Go into Turbotax for 2017.  Open the *.tax2017 file you created for your 2017 taxes.  Turbotax then shows an option to amend your 2017 return.  Follow the Turbotax prompts/inputs/questions.

I know the above. It's a matter of where and what info needs to be amended in the amended return - I'm not finding prompts that relate to overfunding.