need to update ios on 3gs iphone

[Suscyn]

My ipone 3GS is jailbroken and unlocked. the ios is 3.13 (7E18) with BB 04.26.08. I would like to update to a newer ios maybe 4.2.1. I don't know what bootrom it has since I cannot get it to DFU mode or my computer does not recognize it when I go to properties in DFU mode. Also, on CYDIA I get an error of"not able to locate file for coreutilis bin package when trying to upgrade packages. Cydia is 1.1.1 , have ultrasn0w 1.2.1 and cannot upgrade it. Any help would be greatly appreciated.