I marked an order delivered but the customer says the rider has not arrived yet. Can I undo it?

Delivered is almost the end of the road. From Delivered the only move is to Cancelled, and only the owner can do that, because the stamps, the coupon and the review request have already followed the delivery. Between the live stages you can go back, for example from Being prepared to Confirmed. For a wrong Delivered tap, the practical fix is to leave it and note it in your managers' group, since the customer will still get their food. Ask riders to use the link on their ticket so Delivered is pressed at the door. The Orders & delivery tab in the docs explains the statuses.