When HTC first released the One M8 in India, it decided to disable the phone's 4G LTE radio. So despite having a Snapdragon 801 SoC and all the necessary hardware in place, Indian users couldn't connect their phones to LTE networks and enjoy higher internet speeds. The company has been promising to release an OTA update to fix the issue on August 15th, and they've fulfilled their commitment.
The OTA update has started rolling out to the M8 in India, pushing the device to version 2.10.720.1. It brings Android 4.4.3 along with improvements to OpenSSL security and the long-awaited support for 4G LTE. There are many performance improvements, bug fixes, and application updates baked in as well, pushing the OTA's size to a whopping 658MB.
If you don't see the software update notification now, you can check it manually or wait a little bit. The update might be rolling out in stages to all users.
Thanks, Prakash Paruchuri!