How to upgrade my HTC desire 816 with lollipop How can I upgrade my HTC desire 816 to lollipop
HTC is planning to release an OTA for the Desire 816 sometime in the future, but if you can't wait, there is a port of CM12 for your device.
Prerequisites

WinDroid Universal Android Toolkit
Your phone (duh)
Windows XP or higher
USB Cable

Unlock Bootloader

Open the WinDroid Universal Android Toolkit.

Download and install the ADB drivers if it prompts you to.

Choose your device from the list. Wait a few moments while it downloads the correct recoveries.

Turn on your device, go to Settings &gt; Developer Options and Enable USB Debugging and plug it in. If the toolkit does not recognize your device immediately, click on the Refresh button in the bottom right corner. Note: If you are on Android 4.2.2 or above, and you do not see a button for Developer Options, you will need to go to Settings &gt; About &gt; Software Information &gt; More and quickly tap on Build number multiple times until it tells you that Developer Options has been unlocked.

Click the button labeled Get Token ID in the Unlock Bootloader box. This will reboot your device into bootloader, retrieve your Token ID, and open a text file that contains your Token ID and further instructions.

Now that you have your Token ID, you can now submit it to HTCDev, which should have opened in the background when the text file opened previously. If it did not open, you can open the link here. Please login to your account or create and activate one with a valid email address. Once you have done that, go back to the HTCDev link, scroll to the bottom of the page, and paste the Token ID you were given in the previous step. Click Submit, and wait for the email from HTC on the email you created the HTCDev account with.

Once you have received the email from HTCDev, download the attached file in the email named unlock_code.bin. This is the unlock token that can unlock your specific device's bootloader. Once it has successfully downloaded, click on the button named Unlock Bootloader in the bottom left corner. If your device is not already in bootloader, this will reboot your device into bootloader, and flash the unlock_code.bin file. A prompt will appear on your device confirming if you would like to unlock your bootloader. READ THE WARNING COMPLETELY! THIS WILL WIPE ALL OF THE DATA ON YOUR DEVICE!

Press the Volume Up button once, and press the Power button once to confirm the unlock. Your device will now restart.


Your device is now unlocked! You can now proceed to flash a custom recovery and gain permanent root in the next steps.
Flash Custom Recovery

Move to the next box on the right named Recovery and Root.
Download this custom recovery and put it into the folder named Recoveries within the Data folder created by the toolkit. (The file should be named recovery.img.) Now click on the TWRP button. Your device will then reboot into bootloader and flash your chosen recovery.

You are now rocking a custom recovery!
Gain Permanent Root

To gain root access, click the Gain Root button from the tool.

Flash Custom ROM

Download the latest version of CM12
Once you have fully downloaded the ROM you want to flash, find the boot.img file. It should be in the main directory. Copy it and save it somewhere else for later use. Do not extract or unpack the ROM folder, and ensure that you didn't delete the boot.img in the ROM folder itself.
Navigate to the box in top right named Flash. Click on Flash ROM, choose Yes if you are ready, and choose the ROM .zip file you previously downloaded. The ROM you chose will be pushed to your phone. It may take a few minutes depending on the size of the ROM, so please wait until the loading icon has stopped spinning. Once it has finished pushing the ROM, your phone will boot into Recovery. Once you have booted into Recovery, choose Install. Scroll down until you see the ROM .zip file you chose previously. Choose it, and flash it. Once it has finished flashing, go to the Reboot options within the Recovery, and reboot to Bootloader.

You now have CM12, Android 5.0 on your HTC Desire 816!

Adapted from this post