You mean your
W-4--
not your W-2. Your W-4 is the form you fill out for your employer so they know how much tax you want to have withheld from your paycheck. You can fill out a W-4 that will change how much they are withholding, but there is not much of 2020 left in which a change will have a big effect. If you expect to owe at least $1000 to the IRS at tax time you could also make an estimated payment ahead of time on the IRS site to avoid an underpayment penalty on your tax return.
https://www.irs.gov/pub/irs-pdf/fw4.pdf
https://www.irs.gov/payments/direct-pay
If you are a military retiree simply log into your account at mypay.dfas.gov and you can change your W-4 information right then and there. If changed before the 15th of the month it will take affect with your next retirement payment. Change it after the 15th and you won't see the change in your retirement pay for 31-45 days.