I paid my employer 50% back for tuition reimbursement in 2020. They had reimbursed me from 2017 to 2019. Do I adjust the income down when I file the 2018 and 2019 amended tax return?

You should report the amount spent/billed for tuition and fees on the 1098-T, regardless of who paid it. The employer reimbursement amount should then be entered on the Scholarships/Grants screen on the Employer assistance line.

I did that when I filed the original taxes in 2017 to 2019.  Now that I have to pay my employer back, how do I file it in the amendments?  Reduce my income in 2018 and 2019 amendments because they had increased my W2 income in 2018 and 2019 for the reimbursement they provided back then. 

@49Edgemere Yes, you will need to Amend your prior year returns for your repaid employer tuition assistance. 

You may qualify for an Education Credit in those years by doing so. 

Click this link for more info on Repaid Employer Tuition Assistance.