To adjust your billing detail (i.e. Sales Tax or Gratuity) percentage or dollar amount, you will select Settings > Billing Details > Edit. From here, you can adjust the default value or, if you would like to change a particular location's tax rate, you can adjust the overriding value.
Once updated, this new percentage or dollar amount will only affect newly created documents moving forward. For any existing documents that have already been created within the event or booking, you may want to manually adjust.