After the LG G4 recently received a Marshmallow update, the HTC One M8 follows in line. This adds the HTC One M8 in the list of non-Nexus range to receive the Android 6.0 Marshmallow update. Here is a manual guide to install the OTA on firmware. This install can be done in the stock recovery through the Apply Update option. For users with a rooted M8 or have custom recovery installed, you should not try this install as it may brick your device.
How to Update the Official Android 6.0 Marshmallow on HTC One M8
First make sure that all your updates are applied before starting the installation process.
Download the Android 6.0 Marshmallow OTA Update
and save it to the phone’s built-in memory.
Now boot the M8 device to Recovery mode by holding down the volume up, power and home buttons for 4 to 5 seconds. When a bootloader menu appears select the recovery option if available. The alternative you could setup adb, while the device is still turned on and fastboot.
When the HTC M8 is on the Recovery mode, choose the Apply Updates from the device storage and select the OTA file saved. If prompted, select the storage 0 option.
The installation process which takes a few minutes will start on your M8.
Once the installation is complete, the HTC One M8 will reboot automatically. If this does not happen, you can manually reboot your system to start enjoying the Android 6.0 features.