Understand that everything you do is at your own risk and none of these modifications are guaranteed to work by their authors. This will wipe all of the contents of your phone, so consider backing up any files that aren't on cloud storage.
You will need to do the following in this order:
1.) Unlock your device's bootloader,
either through http://www.htcdev.com
or by other means. If you are on HBOOT 1.44 or earlier (unlikely,) you can do this "off the grid." For convenience's sake, you can safely unlock your bootloader via the aforementioned website, but it may have implications on your warranty.
2.) Install a custom recovery
of your choice (CWM or TWRP) and obtain root
on your device.
3.) Create a backup
of your current OS in the event that anything goes wrong. Immediately transfer this backup to your PC/Mac storage.
(These three steps are detailed here
(security off) your device via Firewater, see here
5.) Download either the pre-rooted or non-rooted 4.4.2 Google Play Edition RUU from here
, and be sure your MID and CID (ex. PN07140, T-MOB010) are listed and supported before continuing.
(RUU is short for ROM Upgrade Utility; think of the file as an OS install DVD that you will be putting into your phone, much like you would a freshly formatted computer.)
6.) If your bootloader is unlocked, custom recovery is installed, rooted, backup created, S-OFF, Google Play Edition RUU is downloaded, and you are successfully connected to your phone via adb, continue with the installation steps in the last linked thread exactly as they are listed, beginning with "adb reboot bootloader". Again a reminder that this will wipe your phone's storage and, once rebooted, will be running the Google Play Edition OS.