Installing the Global version of 4.1 firmware on the AT&T-branded One X model will most likely be impossible due to the hardware differences between the different variants of this handset:
[AT&T variant] is 0.4mm longer than the international model, and
features a 1.5 GHz dual-core Qualcomm Snapdragon S4 (MSM8960) SoC with
an integrated on-die LTE (4G) modem instead of the Nvidia Tegra 3 [found on the international model]
which requires an external modem and is LTE-incompatible, and 16 GB of
internal storage instead of 32 GB.
You will either have to wait for HTC and AT&T to release an official version of Jelly Bean for your model, or take the plunge and root, bootloader unlock, and flash a custom ROM on your own if you want 4.1 sooner. Instructions for doing so can be found at XDA-developers forums.