I changed fulfillment status on the wrong order to “fulfilled” . How can I change it back to “Ready for pickup”?
It sounds like you want to unfulfill it. In that case click the 3 dots in the fulfillment order box and cancel the fulfillment.
If you want to learn more about how fulfillment works, here are the docs: https://help.shopify.com/en/manual/orders/fulfillment