It is not possible to restore the EC850 and EC860 to factory settings.
However if you wish to change the quantity coffee delivered simply press and hold the delivery button until the required amount has been delivered then release the button.
If you are setting one of the milky drink buttons you will need to do this twice: Press and hold the relative button to start programming. release the button to stop the flow of milk and program the quantity. Press and hold the same button and release the button to stop the flow of coffee and program the quantity.
To deliver the same drink, simple press the same button once and the newly programmed quantity of milk and coffee will be delivered.