The update brings software version 3.62.401.1 to your handset and adds enhancements to Daydream while also adding OpenGL ES 3.0 support. Sense 5.5 brings some new features to BlinkFeed, adding Instagram and Google+ feeds among other changes. The Gallery is improved in Sense 5.5, which also brings a new UI for Video Highlights.
If you live in the U.K. with your HTC One and have yet to receive the update, don't pout. These updates rollout in waves, so be patient. Your update will come.
The HTC One in the U.K. is receiving Android 4.3 and Sense 5.5
source: @HTC_UK via Pocketlint