# Get the reference to the "vms" service:
vms_service = connection.system_service.vms_service

# Find the virtual machine:
vm = vms_service.list(search: 'name=myvm')[0]

# Locate the service that manages the virtual machine:
vm_service = vms_service.vm_service(vm.id)

# Locate the service that manages the CDROM devices of the VM:
cdroms_service = vm_service.cdroms_service

# List the first CDROM device:
cdrom = cdroms_service.list[0]

# Locate the service that manages the CDROM device you just found:
cdrom_service = cdroms_service.cdrom_service(cdrom.id)

# Change the CD of the VM to 'my_iso_file.iso'. By default this
# operation permanently changes the disk that is visible to the
# virtual machine after the next boot, but it does not have any effect
# on the currently running virtual machine. If you want to change the
# disk that is visible to the current running virtual machine, change
# the `current` parameter's value to `true`.
cdrom_service.update(
  OvirtSDK4::Cdrom.new(
    file: {
      id: 'CentOS-7-x86_64-DVD-1511.iso'
    }
  ),
  current: false
)