I want to update my jailbroken iPhone 3GS iOS 4.1 to iOS 5.1 or any other iOS which is best for it. But I havent synced my iphone since a long time and that device I synced it to is no longer with me. So there's no backup. PLease help me:(
Put your device in DFU mode. (http://www.iclarified.com/entry/index.php?enid=1034 and http://www.ihackintosh.com/2009/06/recovery-mode-and-dfu-mode/). Connect to iTunes and it should prompt you with a box saying that your iPhone is in DFU mode and needs to be restored.
Sometimes it's possible to backup your phone and restore from that one once updated. Will not recommend it though; did that with my iPod and it didn't work perfectly afterwards.