Go to the job with the invoice you want to edit.
Click on the invoices tab.
Click 'View' next to that invoice in the list.
Click 'Edit' on the right side of the invoice breakdown section.
You can edit the information within the invoice. Click in the field with VAT amount > delete the value > select the correct amount from the drop down list.
Please note: Any changes you make to an invoice will not be synced to your accounting package. You will need to manually change the invoice in your accounting package if it has already been sent.