I needed to fill my taxes for 2018, but filled it for 2019 by accident. How can I fix this?

If you used the current online product to enter 2018 information and filed the return and it was accepted, you will now have to prepare, print and mail an amended  return form 1040X to change the 2018 information to 2019 information. 

If your return is rejected by the IRS, you can correct the problem and fix the reason for rejection and re-efile your return. If your return is accepted by the IRS, you must prepare, print, sign and mail an amended tax return form 1040X to correct the problem.

DO NOT change your original tax return until you start the amended return. If you have already changed it from the way it was originally filed and accepted, you must change it back before you start the amended return. Wait until you receive the refund from your original return before beginning your amended return to make sure you enter the correct amount. Here is an FAQ to help you:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

An amended return cannot be efiled. You must print and mail it yourself. TurboTax does not print and mail it for you. In normal times, it takes the IRS about 16 weeks to process an amended return but it is taking longer now since the IRS is not processing mailed returns.

To do the 2018 return, you must buy, download and install on your computer the 2018 program: https://turbotax.intuit.com/personal-taxes/past-years-products

A 2018 return can no longer be efiled. You must print and mail it yourself. Use the filing instructions printed with your return for what to include and where to mail it. 

It depends.

In regards to the 2018 tax return you prepared using the online software for 2019, as long as you have not filed the 2019 return using your 2018 information, you can use the following link to clear and start over your 2019 tax return.

Clear and start over your return

If however, you used the online program to file your 2018 tax return, you will need to prepare an amended return for 2019.  Please see the following link for instructions on how to amend your 2019 tax return.  Be sure to report only your 2019 tax return activity in the 2019 online tax program.

How to amend my return

I have attached a link with instructions on how to obtain the 2018 program so you can prepare your 2018 tax return.  You will need to prepare your 2018 tax return using this program to report all activity for 2018.

How to prepare a prior year tax return