If you go to taxes>prepare return>you will see an adjust option available for most of the boxes.It will bring up a pop up box when you hit on the word adjust.The adjustment account needs to be changed away from a bank account.(usually it would be best to create a liability account especially for vat adjustments-do not use the existing vat control/suspense accounts)
If it is an increase adjustment type in the figure.If it is an adjustment to reduce the box type in a minus symbol and then the figure>save
It is worth confirming with your accountant that they are happy with you doing that this way.