The VAT amount on an invoice is wrong
(If you've already recorded the transaction, see Fixing errors in a sales transaction.)
If the VAT amount is wrong, first check to be sure that the correct VAT code has been selected for each line item. If a code is incorrect, click the arrow next to the VAT code and select the proper VAT code from the list that appears.
If carriage is taxable, be sure the correct VAT code has been selected for it, as well.
Finally, if these settings are correct and the VAT amount still is wrong for some reason, click the arrow that appears to the left of the VAT amount. The VAT Information window appears and you can change the VAT amount for each VAT code used on the sale. Make your changes and click OK.
If you've previously made changes in the VAT Information window, you can undo those changes by displaying the VAT Information window again and clicking the Recalculate button.