If plus has already been added, as long as you have
not filed, paid, nor registered you can remove Plus.
https://ttlc.intuit.com/replies/4241405
You can also delete what you have entered and start over. Here's how (provided you haven't already filed, paid or registered your Turbo Tax product.
https://ttlc.intuit.com/replies/4248526
It's also a good idea to
make sure Turbo Tax Free is right for you. Let's see if you meet the criteria
for the Turbo Tax Free editon:
https://ttlc.intuit.com/replies/3334660