You can un-allocate any allocated lines that are not yet marked as Delivered. This includes lines on Orders that have been already submitted to the Warehouse (but this is not recommended as the Warehouse will not receive the changes).
Once you have un-allocated the lines the status of the lines will change back from C to O and you can update them and re-allocate them in the Allocate Orders screen.
Once a line on an Order is marked as Delivered in VC, it will not show in allocations, cannot be unallocated and cannot be updated.