How do I go back to State return to add my info for 2020 so I get the Oregon kicker?

Have you already e-filed your return?    As long as you have not, you can open your return, click on State from the left side menu and go through the Oregon return until you get to the Oregon Kicker section.     If you used TurboTax last year, your 2020 info will be there already.    If you did not, you will need info from your 2020 Oregon return to claim the credit this year.  

To calculate the amount of your credit, multiply your 2020 tax liability before any credits—line 22 on the 2020 Form OR-40—by 17.341 percent.  This percentage is determined and certified by OEA.  Taxpayers who claimed a credit for tax paid to another state would need to subtract the credit amount from their liability before calculating the credit.   

TurboTax will calculate the credit, but you will need to enter the Line 22 amount if it is not auto-populated from last year.  

See Oregon DOR.

If you already filed, you will need to amend it.