If you're working in Receipts and Payments, then you can simply correct the amount by navigating to menu Setup → Bank Balances and amending the amount against VAT. If you are due a VAT refund from HMRC, enter the figure as a negative (minus infront of the value).
If you're working in Income & Expenditure (I&E), then the opening balance will have been brought forward from the previous year, where it would have made up some (or all) of the difference between Box 7 and 8, and appeared on the balance sheet as a debtor/creditor. Therefore, changing this opening balance will affect your prior year and mean that your Box 7 will not equal your Box 1.
If the opening VAT balance needs correcting, this will either mean the prior year needs restating or that correcting transactions will be required. Please contact support for specific guidance on this.