Have you ever updated through it on any htc phone???
HTC phones normally get a firmware updates OTA. I've updated my Desire last year from Android v2.1 to v2.2 OTA. All data was preserved.
More than 15 days n I havent rooted my sgs im too much confused n afraid or rooting n flashing
Take it easy. Read and understand as much as you can.
Know what you are doing. There is no rush.
Only after you feel comfortable enough, go ahead with rooting and flashing custom ROMs. These will only get improved in the coming months.