Logon to the console of the XenServer host. Check host uuid:
# xe host-list

Change the name:
# xe host-set-hostname-live host-uuid=<HOSTUUID> host-name=<NEWHOSTNAME>

Example
# xe host-set-hostname-live host-uuid=00cbeec7-c229-4ab9-990f-cdb5f781da11 host-name=xen-002

Reboot the host:
# reboot