It depends. If Apple is still signing 4.2.6 you can just use Shift+Restore (or Option+Restore) to point iTunes to the correct ipsw. If Apple is NOT signing it, you can't use it unless you saved your SHSH earlier. If none of this means anything to you, then please read the sticky posts about how to jailbreak and/or google some walkthroughs about it. It will be much easier if you've watched a few YouTube videos on how to do it.