How to upgrade iOS version for jailbroken iPhone 3?

Hi there, Can someone please send me a tutorial or some kind of steps that I need to follow in order to upgrade the iOS version of my iPhone? My current version is 4.2.1 (8C148), Model MB046LL and the Firmware is 06.15.00. The phone was jailbroken, however I do not know what version of redsn0w they used. Please let me know if additional information is needed. Thanks in advance.