If your income changes during the year and you realize that your previous estimates may have been inaccurate, you can simply adjust your estimate accordingly in your next quarterly filing. For instance, if you underreported in the first quarter, you can increase your estimate in the second quarter to make up for the initial shortfall. At the end of the day, estimated taxes will always deviate at least somewhat from your actual tax liability, so it is normal for these kinds of adjustments to be made.