How to undo a software update iphone 4?

I Have An Iphone 4 iOS 6.1.3 And Want To Jail Break, How Do I Go To 6.1.2? Or Anyway To Jailbreak 6.1.3

Impossible to do, you shouldn't updated it in the first place so you would have 6.1.2 by then

You don't

you put it on iTunes
and it (re-)loads the current S/w version. That's it. (sorry)

Restore from a previous point, beware though

If you have an iPhone 4, then yes you can jailbreak but it will be with redsn0w only! And it will be a semi-tether jailbreak which means when you reboot, turn off your phone or it dies your phone will work but only some apps! And then later when your on your computer you can boot your phone back up with redsn0w and everything will work: all apps, Cydia, tweaks etc. So there's no way to downgrade iOS software but you can jailbreak iOS 6.1.3 is you have iPod4, iPhone 4 and iPad mini I think. Look up a video on how do a 6.1.3 jailbreak, it's really easy and doesn't take a ton or time.