This is configured in PI Collective manager.
Note that client's automatically update the configuration when they connect to one of the collective members but if the old servers where removed before they connected you have to manually remove and re-add the collective (or connect to any of the new nodes manually) on each client.
Open PI Collective Manager
Edit the FQDN field (this can contain either the FQDN or the IP address to the member)
Andre Johnsen already pointed you to the collective manager which is indeed the way to go about it. You may also want to take a look at https://techsupport.osisoft.com/Troubleshooting/KB/KB00181 if you need to point some clients or interfaces to the new servers.