VM or appliances?
At a high level (you can get all the nitty gritty details from the links that Marce liste)
If VMs, then Cisco recommends a restore config to new VMs. The upgrade is a simple case of deploying 5 new VMs and leaving them at the "setup" prompt stage. One tip: you can now deploy an 8vCPU PSN that consume fewer resources for PSNs that don't need too much horsepower. Make a config backup. Export all the Admin certs of all the nodes and keep them aside. Export all the trusted CA certs you may need. Export Portal and EAP certs as applicable.
Shutdown the secondary PAN and then run the setup on the first VM using the details of the Secondary PAN. Patch the node.
Restore the ISE 3.0 config backup on it. Install the Admin cert. Join node to AD if applicable. Check that the "upgrade" worked and that all configs are looking good.
Then power down the first PSN, and run setup on another VM using the details of the PSN. Patch the node and then install the Admin cert. Register PSN to the new ISE 3.2 PAN.
Rinse and repeat for the rest
Last node to go is the old ISE 3.0 Primary PAN.
Finally, promote the ISE 3.2 Standby PAN back to Primary.
PS: Having said all that, I have started using the inline upgrade mechanism in ISE for 3.2 to 3.3 upgrades and it's been an absolute pleasure. None of the above steps are required. All you need is the ISE 3.x upgrade bundle and patch file - put that on your repo, and then follow the wizard in the ISE GUI. It take a little longer, but the beauty is that you can't stuff it up. ISE will check along the way and allow you to break the upgrade into phases - as many as you need. At end of each phase you can pause and check the status. And best of all, no need to make new VMs, install certs, join AD, fiddle with the CLI, etc. - the only time you should need to rebuild a VM is if you are changing the size of the hard disk (e.g. going from 200GB to 300GB or whatever)
