You make an "amended" W-4 to correct withholding from the time you submit the W-4 on - for the future. It is not retroactive.
What was withheld for 2015 cannot be changed - it is what it is. Your tax due (or refund ) is your tax liability minus your withholding (right amount or not). If the withholding was more than the tax liability, you get a refund, if less, you owe additional tax.
Next year (this year 2016) keep track of your pay stubs and the amount of withholding and talk to your employer if it is incorrect. You are responsible for paying the tax, and making sure the correct amount was withheld even if the employer made an error. That is what pay-stubs are for - for you to review and alert your employer to any errors - not to wait until the year is over and done with when it can no longer be corrected.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
