Only orders that are not filled and are still live can be edited.
Tap on Order Manager on the app home screen as shown below.
Locate the order you want to edit in the Order Manager.
You can edit the price or quantity by tapping the "Edit".
After you have completed the "Edit Price" or "Edit Quantity" tap the "Save & Apply".
Tap Yes to confirm or No, go back to return to the previous screen.