HTC One M8 features a 5-inch 1080p display, 2.3 or 2.5 GHz Snapdragon 801 CPU/GPU, 2GB of RAM, enhanced “BoomSound” speakers, a 2600mAh battery, up to 32GB of internal storage and support for external microSD cards up 128GB. One of the most unique features of the HTC One (M8) is its new “Duo Camera.” This camera features two sensors: one Ultrapixel sensor and one depth of field sensor to allow users to select the image focus after the picture has been taken.
HTC One M8 has received the Android Lollipop update but not the latest Android 5.1. It may take a while for One M8 to get this update as it will be behind One M9 in the pecking order. Till then you can use the CM 12.1 custom ROM. Follow this guide.