thanks bro for your advice, successfully update to GB. very easy when using Kies to update the OS version. install Kies from official Samsung web, change your USB setting in your phone (something like connect via KIES), connect your USB to your phone, then it will pop up the message, asking for upgrade.. no backup contact, no backup games, no backup sms, nothing.. click, proceed and wait until the process done. then phone will restart and walllaaa, new version sudah ada..

can feel the differences. startup is faster, response is faster, some simple game loads faster and seems battery life improve little bit. But occasional laggish and hang still happen, when open games, youtube, fb and wechat (all together). Expected I guess for single core phone..

I think for now, this phone gonna stand for another 1 year, hahaha.