Unfortunately, no. Once you've confirmed your selection, it cannot be changed.
When you confirm your choice, the payment is processed automatically and directly to that supermarket or retailer. Because supermarkets treat these as final sales and don't offer refunds on gift cards, there is no way to pull those funds back once the voucher has been issued - even if the choice was made by mistake.
This is why the process includes a confirmation screen with a clear "You cannot undo this" warning and a toggle switch before you finalise - these are there to make sure the choice is definitely right before the transaction is locked in.
Before you confirm - check your link expiry
Your voucher link has an expiry date, shown on screen when you first open it. You need to confirm your brand selection before this date - after it expires, the link will no longer work.
Within that time, take as long as you need. Read the description under each brand carefully - it tells you where the voucher can be used and any restrictions that apply. If you want more detail before deciding, the Gift card and voucher T&Cs section of this help centre has full terms for each brand.