Update: It appears this RUU may be improperly labeled, as we're hearing it fails to flash on a number of CIDs which it should be compatible with.
If you're the owner of a European model HTC One X, you're probably chomping at the bit to get your update to Android 4.1 Jelly Bean. While that update has begun rolling out in parts of Asia, it has yet to show up anywhere else. Today, though, HTCRUU published an official RUU file (a full, flashable ROM) for the One X in its unlocked European flavor - software version 3.14.401.27, based on Android 4.1 with Sense 4+.
First off, this is only for specific variants of the One X - this won't work on an AT&T model in the US, for example. Your device needs to have the proper CID number (a hardware identifier code) in order to flash the update. Luckily, there are quite a few CIDs this particular RUU is compatible with:
Figuring out your CID is pretty easy. Just load your One X into fastboot mode, hook it up to USB, and run "fastboot getvar cid" to find it. Once you download the firmware, boot your phone up again with USB debugging enabled in fastboot mode (make sure you have the right USB drivers), run the exe file, and follow the instructions. This post guides you through the basic process.