It doesn't matter if you file your amended return prior to filing your 2019 tax return. However, I would suggest that you file your amended return as soon as possible to avoid paying additional penalty or interest fees as a result of the late filing, if you owe money.

If you filed using Turbo Tax Online version, here are your steps for amending your 2018 return.

1. Sign in at turbotax.intuit.com/ with the user ID you used when you filed your 2018 return. 
2. Select Show next to Your tax returns & documents.
3. Select Download .tax file. You'll need this to amend your return..
4. Once you've got your .tax file, select 2018, and then select Amend (change) 2018 return.
5. In the expanded instructions, select download TurboTax Desktop 2018.
6. Install the TurboTax 2018 software depending on the type of computer you are using.
7. Windows instructions
8. Mac instructions
9. Now open your .tax file in the TurboTax 2018 software by choosing File and then Open Tax Return (Windows) or TurboTax and then Open Tax Return (Mac). Browse to the location of your tax file and select Open.
10. You're ready to amend your 2018 return.     

To add your W2C to your opened desktop program, please perform the following steps.

1. Go to federal taxes

2. Then go to wages and income

3. You will arrive at a screen that asks you how you want to enter your income, please select

    I will choose what I work on.

4. Then go to Wages and Salaries and select Start

5. Enter your W2C statement information as a W2

6. After the information has been entered, you will arrive at a screen that asks if any of these uncommon situations apply to this W2 and you will check the box that indicates this is a corrected W2.

At some point, in filing this amendment, the program will ask you why you are amending your return.You will need to state that you received a corrected W2 in the mail. When finished, we'll give you instructions for printing and mailing your amended return as amendments can't be e-filed.