The child tax credit amount hasn't changed, they are just paying you some of it now instead of with your refund next year. If you want to adjust your withholdings to fix your situation, the adjustment will need to add up to however much you will receive in advance payments. You can use the "extra withholding" section in step 4 of the W-4 to specify the exact amount of additional withholding you want. Please note that this amount is per pay period, so you'll need to divide the total amount over the number of remaining pay periods in the year.
https://www.irs.gov/pub/irs-pdf/fw4.pdf
Nick K, EA
