Following are the steps that you can follow to update your XenServer host from version 7.1 cumulative Update 1 to version 7.1 cumulative update 2:

1- Go to the Citrix XenCenter and select the “Pool” option from the menu bar that is present on the left side of your screen.
2- Now tap on the “Tools” option that is present on the menu bar.
3- After that, tap/click on the “Install Update” option.