How do I update my W4 so I don't owe taxes? Do I just advise my employer to take out more money? How do I estimate how much so that I do not owe any or very little?
You can use the IRS Tax Withholding Estimator to estimate the extra amount you should withhold. Divide the amount among your remaining pay periods for the current year and note that amount to Form W-4 Line 4(c). It will add to your Federal withholdings on your W-2. If you have too much withheld, it will be refunded on your tax return.