Best way to adjust W-4 and California DE-4 for Alimony deductions?

I have a divorce agreement that was executed in 2017 (prior to tax law change re: alimony deductions) and was recently amended so that I am paying more alimony than I was previously. In addition, my salary has changed. I would like to amend my W2 and state of CA DE-4 so that I don't end up owing or having a big refund from either federal or state. Can you recommend the best way to do this? The alimony deduction and a credit for one dependent are my only main considerations. I file as head of household. Thank you!