What you claim at work on the W4 doesn't have to match your actual tax return filing status. It's just to tell your employer how much to take out. You will get credit for all the withholding on your tax return. I'm married but claimed Single on the W4 to have more taxes taken out.
If you had claimed married you would have had less taxes taken out and would have a smaller refund or owed a tax due. There is nothing to fix or amend. If you change your W4 to married you may be surprised at the end of the year when you file your tax return and don't have as much refund.
