You can always modify the price of an individual order by selecting the HELP button in the top right section of the order info screen.
1. Tap "PRICE ADJUSTMENT"
2. Adjust the cost and tax rate of the order
3. Provide reason for the adjustment
4. Tap "CONFIRM"
If you would like to refund an item in an order after the food has been picked up by a delivery partner, you can do so by reaching out to Uber support.