HTC recently rolled out the Android 6.0 Marshmallow update to Desire 816 with software version 3.12.401.4.The update comes to the device with a TWRP flashable official HTC firmware. It brings a brand new HTC Sense 7 user interface and Marshmallow firmware with build number 3.12.401.2. It is a pre-rooted ROM and comes with all the features from HTC firmware. As for now, the ROM works for only single-SIM variants and for the HTC Desire 816 A5 UL variant only. But with more development, dual-SIM support can be expected.

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[How to] install Android 6.0 Marshmallow on HTC Desire 816 using TWRP recovery.

1. Download and transfer Android 6.0 Marshmallow from  download section, you wish to flash to your device’s storage.

2. Boot your device into TWRP recovery.

3. Select “Wipe” from TWRP main menu and do a “Swipe to Factory Reset” on the bottom of screen.
   ? Note: This will factory reset your device.

4. Go back to TWRP main menu, tap on “Install” and select the Marshmallow ROMs .zip file that you transferred to your device in Step 1.

5. After selecting the .zip file, do “Swipe to Confirm Flash” on the bottom of screen to begin the Marshmallow ROM installation process. This may take some time.

6. Once your ROM is successfully flashed, you’ll see “Wipe cache/dalvik” option, select it and then do “Swipe to wipe” on bottom of screen.

7. Once cache & dalvik wipe completes, hit the “Back” button on-screen and then select “Reboot System” button.

That’s all. Your device will now reboot with the Marshmallow ROM you just flashed. Since this will be the first reboot after flashing a new ROM, it might take some time to rebuild cache and finish boot, have patience.