You can make corrections to already posted VAT entries and change the total sales or purchase VAT amounts without changing the VAT base. You may need to do this, for example, if you receive an invoice from a vendor that has calculated VAT incorrectly.
Even though you may have set up one or more combinations to handle import VAT, you must set up at least one VAT product posting group code (you can name it CORRECT, for example) for correction purposes, unless you can use the same G/L account in the Purchase VAT Account field on the VAT posting setup line.
Open the 
General Journal
window.
Fill in a journal line for the correction. Enter the corrective amount in the Amount field. Note that the VAT Base Amount field remains 0.00 (you cannot enter anything in the field).
Click Posting, Post to post the journal.