The latest version of HTC's custom Sense UI is now available for select versions of the HTC One M7. More specifically, the HTC One (M7) Developer Edition and the unlocked models are now able to install Sense 6.
It's worth pointing out that this isn't exactly the full version of Sense 6 that currently powers the HTC One M8. Notably absent from the software build are advanced DuoCamera effects and Motion Gestures. The reason, as HTC Source points out, is because of hardware limitations.
The software update weighs in at 667MB so be sure to have plenty of battery available on your phone. What's more, you'll want to connect to a Wi-Fi network lest you want to chew through your data plan.
If you have an HTC One M7 Developer Edition or an unlocked model you can search for the update, simply navigate to Settings > About Phone > Software Update > Check Now.
As for the carrier branded versions of the year old HTC One M7, they look to be in the final stages of certification.