HTC Desire 816 CyanogenMod 12 is a custom ROMs for all variants of Desire 816 devices which gives you early taste of Android Lollipop. The official HTC builds for certain devices started rolling out but not yet available for most of the devices out there. But developer Garak has ported the CyanogenMod 12 Android Lollipop for all variants HTC Desire 816 and this is an official NIGHTLY build. Meaning you will get daily NIGHTLY updates via OTA, so no manual download and update.
The CM13 Android Marshmallow ROM is also up for Desire 816. If you are interested then you can visit the below link for Desire 816 CM13 Marshmallow ROM page: HTC Desire 816 CM13 Marshmallow ROM
DISCLAIMER: Installing custom ROM requires a custom recovery to be installed on your device, which may VOID your warranty and may delete all your data. Before proceeding further you may consider backup your all data. Understand that YOU are the only person responsible for any disaster that may happen to your device. Neither we nor the developer can be held responsible for YOUR mistakes.