Simple. If you're stock like mine was with S-ON
1. Take the zip file. Rename to 'PH85IMG.zip'
2. Place file in Mobile card aka ext_sd (Not HTC storage)
3. Check battery is at least 50% charged.
4. Remove battery, replace battery.
5. Hold power AND volume down button to reboot.
6. Phone will go into boot mode and find the new file, wait for it to extract it. Follow screen instructions
7. Phone does a quick check of file contents then shuts off.
8. Hold power to reboot.
9. File is unzipped and update occurs.
10. Phone asks you to reboot, volume up for yes.
11. Enjoy ICS 3.6!
Also, since I was not rooted, I wanted all the same settings/home screen/apps/data. I used MyBackup Pro and everything went flawlessly. Just make sure the file contents are backed up to your external card and not internal. Also copy MyBackup pro app to your SD. Once updated your internal files are erased!!! I was paranoid and just backed up my HTC Storage folder to my computer too, but I'm pretty sure it's not necessary
---------- Post added at 08:37 PM ---------- Previous post was at 08:32 PM ----------
Also, I haven't had any issues except with Facebook contact updates. Just use the Syncmypix app. And a few apps from GB wouldn't install, they were minor and old, but I assume this is from lack of ICS support.