As was mentioned, retirement contributions through your employer are already factored in to your W-2.
If your income is getting too high, contributing to a Traditional IRA may be easier than adjusting your employer retirement. You can contribute to a Traditional IRA as late as the regular due date of your tax return (usually April 15th, but this year it is July 15th). That way, you can contribute the exact amount that you need to qualify for the Premium Tax Credit.
