You can edit existing reservations by:
- Go to the Reservation List (Inventory -> List Reservations)
- Click on "More Actions" for the reservation you want to edit
- Click "Edit"
- You can then:
- Change the dates
- Add or remove items
- Re-submit the reservation
and the changes will be made. Note that you can allow users to edit their own reservations as well by enable the option in Settings -> Loans & Reservations.