If line 66 is less than line 58, subtract line 66 from line 58 and enter the difference. This is the amount of tax you owe, prior to any penalty or interest that may be due.
About Iowa income tax withholding...
If the amount you owe (line 70) is large, you may wish to check the Withholding Estimator to estimate your recommended withholding. You may file a new IA W-4 with your employer to change the amount of Iowa income tax withheld from your pay.