Can I override income tax for all employees at one go?

Yes, you can import the income tax amount for all employees while processing the pay run. To do this:

Go to the Pay Runs module.
Click Create Pay Run or View Details if you already have a draft pay run.
Click Import/Export > One-time Earnings and Deductions.

One-time Earnings and Deductions

Upload your CSV or XLS file containing Employee Number, Income Tax and Income Tax Overridden Reason. You can view a sample file to make sure that you’ve entered the data in the correct format.
Select the Character Encoding.
Click Next.

Select File

Map the Zoho Payroll fields with the columns in your import file.
Click Next.

Map Fields

View the import preview and click the Import button.

Income Tax Import

Click Apply Changes in the payroll draft page.

Income tax amount for the employees will be updated in the pay run.