Here's the complete guide from xda-developers:
http://forum.xda-developers.com/showthread.php?t=778880
It still took me quite a while to figure out how to flash the phone, there are quite a few little quirks. Here is the process I followed:
- Install the Samsung Kies application (comes with the phone on a CD), back up all your contacts, calendar, etc.
- Use the z4root easy root app.
- Install Titanium Backup for android, requires root access.
- Run Titanium Backup and back up all applications and settings.
- Shut down your phone.
- Hold the Home, Menu, Volume Down and Power keys until a "download" screen appears.
- Connect the phone to your PC via USB.
- Follow the instructions from xda-developers on updating the firmware for the I5800 (Europe firmware seems to work fine for New Zealand phones).
Once you've completed the updating process you should be able to restore any apps and settings from the titanium backups.