iPhone 4 Won't Update to iOS 5!!!

Hey guys, I've been trying to update my iPhone 4 to iOS 5 and I'm not having any luck. My iPhone is jailbroken and when I go to update it, it says backup error and that I will lose all of the contents of the device. I don't want to restore because I have lots of things on my phone I'd like to keep without all the trouble of having to put them onto my computer and then back onto my phone. I know that it's possible to upgrade without restoring because my cousin with the exact same phone, same firmware, and jailbroken did it without a problem. Please tell me how or if I can get pass this error. Thanks!