You have a problem. A 2020 tax return cannot be filed with tax data from any other tax year except 2020. If you are lucky the IRS will reject the e-filed tax return and then you can remove all the 2018 tax data and enter Only 2020 tax data then e-file the 2020 tax return again.
If the IRS accepts the 2020 tax return that was filed then you will have to wait for the IRS to send you a letter questioning what you reported on the 2020 tax return.
You will now have to amend the 2020 tax return and enter your 2020 tax data.
An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 16 weeks or longer to process an amended tax return.
See this TurboTax support FAQ on amending a tax return originally completed and filed using the 2020 TurboTax editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/01/27439
To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2018 tax return can only be printed and mailed, it cannot be e-filed.
