I just learn today... hope my following sharing will help you to get your
GN upgraded to latest firmware - 2.3.6
If your current firmware is 2.3.5 and is rooted, then you just need to
update to the latest firmware 2.3.6 through OTA (Over-the-air) by
Settings > About Phone > Software Update > Update.
This approach will not wipe your phone and your previous root will
still maintain.
Above approach also applicable if your existing phone not rooted and just want o upgrade to 2.3.6
If your current phone is not rooted and under 2.3.5 and want to upgrade
to latest firmware, then better you get your phone rooted under 2.3.5
then upgrade to 2.3.6 through OTA.
Hope above help.
Are you sure with your statement above? Uncle benny said updating via kies or ota will lose root..