I have ipad2 with ios 5.1.1 and already jailbreaked, i want to update itu to ios 6.1.2 because i don't want to ios 6.1.3 that still can't be untethered jailbreak... Is it possible? How?
No it's not possible. The only iOS that you can selectively restore to on the iPad 2 (assuming you have both iOS 4 and 5 blobs) is to the iOS 5 that you have blobs for. If you upgrade, your only option is 6.1.3, which is non-jailbreakable
And just to clarify - iOS 6.1.3 cannot be jail broken at all on the iPad. The only iDevices that can do a [tethered] jailbreak are the iPhone 4 and older phones. It has to do with the processors (and a boot rom exploit) and the iPad 2 doesn't have the right one. So, if you upgrade your iPad to iOS 6.1.3, that's it, jail break's gone.