You'll want to enter VAT adjustment from the box 7 of your VAT return. I'd recommend consulting with your accountant to ensure you record it accurately. Here's how to enter a VAT adjustment.
In your QuickBooks Online (QBO) account:
Go to the Taxes menu.
On the VAT page, select Prepare Return for the period you are submitting for.
Click Adjust in the VAT line that you want to change.
In the Amount section, enter the amount of the adjustment.