You can always modify the sales tax rate on a single charge by going to Admin > Manage > Charges, finding the charge in question, and editing the default tax rate on it.
Keep in mind that if your state or city tax rates change, you are also able to bulk-change your rates for all impacted charges. Go to Admin > Manage > Charge list, and on the right select the button to "Bulk Update Tax Rates":
This will pop up a window that allows you to select which rates you want to change (if you have multiple tax rates):
Set the new rate and it will apply to all charges that used the selected rates.