Is it possible to edit the Social Security or Medicare withholding amount on a payroll check after it is processed?

Thanks for reaching out to this forum, dk1111. I'm here to help you manage employee withholdings for your payroll tax deductions for your payroll tax deductions in QuickBooks Online. 

Since QuickBooks calculates taxes automatically, it isn't possible to edit Social Security and Medicare withholding amounts on your employee's payroll checks once being processed. To help you manage and resolve incorrect withholdings, it's best to void the processed payroll checks and recreate them with the correct tax withholdings. This will ensure accurate calculations.

That said, you can open the Paycheck list to view and void the paycheck that you've processed. Here's how:

1. Go to the Payroll menu and select the Employees tab.
2. Click Paycheck list.
3. Select the paycheck you'd like to void, then click Void.
4. Choose Yes to confirm the action.

Then, you can run your employee's payroll and recreate the checks with the accurate tax withholdings. Once done, review if the Social Security and Medicare withholdings are calculated correctly. Please see below the current tax rates for each tax type:

However, if you're unable to void those checks, I'd recommend getting in touch with our payroll support team. This way, we'll be able to pull up your account and void the transaction for you. Here's how to connect with us:

1. Go to the Help icon and choose the Search tab.
2. Select Contact Us.
3. Describe your concern and click Continue.
4. Choose between Chat with us and Have us call you.

Furthermore, I'm including this reference to help you manage payroll tax filings and payments: Set up QuickBooks Online Payroll to pay and file your payroll taxes and forms.

I'll always be around on this thread for any additional queries when managing employee payroll in your account. Feel free to leave your reply below. I'll make sure to provide further assistance. Keep safe and have a good one!