Are you referring to a downloaded bank transaction that you need to add a tax? If so, we can create a tax adjustment. This option allows you to adjust the amount in the tax payable account to correct tax liability.
First, you'll need to set up an account for tax adjustment. It should be an expense account if you need to increase your tax due. Once done, please follow these steps:
Go to the Taxes menu, find the period you need to adjust, and then select View return.
Click Add an adjustment and the Reason for the adjustment.