The VAT amount on your historical purchase is wrong
If the VAT amount is wrong, first check to be sure that the correct VAT code has been selected. This code appears to the left of the VAT amount. If it's incorrect, click the arrow next to the VAT code and select the proper VAT code from the list that appears.
If the VAT code is 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 purchase. 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.