A virtual network can only be deleted when it is not in use. First, you need to delete all virtual machines that are a part of the virtual network. Microsoft Azure does not have a feature to move virtual machines to another virtual network at this moment.

If you want to keep the instance, delete the virtual machine and keep the virtual disks. Then, create a new virtual machine by using the existing virtual disks. If a gateway is part of the virtual network, delete this as well. Then, the virtual network itself can be deleted.