tinyumbrella::
http://www.ijailbreak.com/ipad/download-tinyumbrella-5-00-07-added-ios-4-3-5-support-and-optional-iphone-4-baseband-update/

and check this link!!! this might help you

http://www.redmondpie.com/how-to-update-iphone-3gs-3g-baseband-to-06.15.00-with-redsn0w-0.9.6b5-for-ios-4.2.1-unlock/