there can be two reasons to change the pool master of a Xenserver
either your pool master is not responding or you just want to change the master.

If you need to change the pool master in Xenserver because pool master is dead or not responding. you need following

Login to any slave (pool master root password will still be used)
issue following commands.

xe pool-emergency-transition-to-master
xe pool-recover-slaves

Output of above commands will be some thing like this.

[root@xen-02 ~]# xe pool-emergency-transition-to-master
Host agent will restart and transition to master in 10.000 seconds...
[root@xen-02 ~]# xe pool-recover-slaves
70e97800-98d5-4a80-a7d0-9032336581b0
d098fb00-3f20-4930-a750-a09dcd993c00
a0dc57c0-7179-4aa0-ac80-801bbadb5f60
e0d4ab10-9e13-4a10-ac60-d07943d21ff0

you are done,

now if you just need to change the master for any other reason, for make sure you disable high availability (in case you have enabled HA)

xe pool-ha-disable
xe host-list
xe pool-designate-new-master host-uuid=<new-master-uuid>
xe pool-ha-enable

Here is some output of changing the pool master

[root@xen-02 ~]# xe host-list
uuid ( RO)                : 00d11112-7017-40ce-90da-b6be7863ecf0
          name-label ( RW): xen-02
    name-description ( RW): Default install of XenServer


uuid ( RO)                : e0d4ab12-9013-401d-a06f-d07943d21ff2
          name-label ( RW): xen-05
    name-description ( RW): Default install of XenServer


uuid ( RO)                : a0dc57c8-7079-40a6-a081-801bbadb5f60
          name-label ( RW): xen-04
    name-description ( RW): Default install of XenServer


uuid ( RO)                : d098fb08-3020-403c-a057-a89dcd993c00
          name-label ( RW): xen-03
    name-description ( RW): Default install of XenServer


uuid ( RO)                : 70e97801-90d5-4081-a0d1-9032336581b0
          name-label ( RW): xen-06
    name-description ( RW): Default install of XenServer


uuid ( RO)                : 805223cc-20d4-4074-b0ba-f02a071fd820
          name-label ( RW): xen-01
    name-description ( RW): Xen 01

xe pool-designate-new-master host-uuid=d098fb08-3020-403c-a057-a89dcd993c00