Hi melissa9,

Yes, you can modify the withholding amount by editing the paycheck.

Here are the steps:

1. On the left panel click Payroll, and then click the Employees tab.
2. Click the employee's name and select View paycheck List.
3. Click the Net Pay link and click the Edit link located at the bottom.
4. Click the Employee Taxes drop-down arrow to expand, and then edit the Federal Income Tax amount.
5. Click OK.

Note: QuickBooks Online calculates federal income tax withholding in accordance with IRS Publication 15 (Circular E), Employer's Tax Guide. 

You can also check the Personal Income Tax section in this article: Payroll 101.

Feel free to reach out to us if you have any additional questions about QuickBooks. Thanks.