When your order is delivered, within 14 days click on my orders, then view order details, in the box of options on the right one is price adjustment, this is me doing it via a web browser, it might be different to get to on the app.
If the price has dropped between ordering and delivery they will refund the difference, either direct to your account as a credit, or you can tick the box to refund it to your bank, this take a few days longer, even on small orders there can be few bob difference, so always worth clicking on the box and they will tell you if there is anything to come back