While CyanogenMod team is busy working on the official Android 5.1 Lollipop based custom firmware for HTC One M8, you can already flash it on your device thanks to an XDA member. This is an unofficial version so please don't go forward if you don't have any previous experience of using all sorts of custom ROMs. Here are the steps to update HTC One M8 to Android 5.1 based CM 12.1 custom ROM.

Prerequisites to update HTC One M8 to Android 5.1 CM 12.1:

Backup any important datafrom your HTC One M8 as a complete wipe will be required during the flashing process.Install USB drivers for your HTC device. Check out this link.Enable USB debugging option on your device:Settings>DevelopmentOptions>USB debugging.Ensure that your HTC One M8's bootloader is unlocked and that you have a recovery tool like TWRP or CWM.Your device must be charged to at least 80% of power.

How to update HTC One M8 to CM 12.1 Android 5.1 lollipop:

Required files: download CM 12.1 Android 5.1 Lollipop custom firmware for HTC One M8 fromhereand extract the boot.img file. Also download the GApps package for your device.

Here are the steps to help youupdate HTC One M8 to Android 5.1 basedCM 12.1custom firmware:

Connect your HTC Oneto your computer using a USB cable and copy the ROM and GApps zip filesto your phone’s SD card root folder.Copy / paste the boot.img file to the Android-sdk-windows\platform-toolsdirectory.Disconnect theUSBcord andpower off your HTC device.Boot your device in the Bootloader mode by pressing and holding theVolume DownandPowerkeys.Once in the Fastboot mode, navigate toandroid-sdk-windows\platform-toolsdirectory on your computer and open command prompt withShift + Right-clickcombo and click on theOpen Command Prompt.Type the following string into the window:fastboot flash boot boot.imgand press Enter.Fastboot reboot-press Enter again.Follow the onscreen instructions and selectBootloader> go toRecovery.Once in recovery, perform wipe data task: from CWM recovery menu, selectwipe data / factory resetand confirm the process.Also performwipe cache partitionfollowed bywipe dalvik cache(from Advanced).From recovery main menu, selectinstall zip from sdcardfollowed bychoose zip from sdcard.Locate the CM 12.1 Android 5.1 custom firmware file and confirm the flashing process.Once the ROM is flashed successfully, repeat the above step but select GApps package this time.Return back to main recovery menu and reboot your HTC One M8 into the new custom ROM. First boot takes a good 5 minutes so please be patient.

Enjoy the new ROM with fresh looks of Lollipop.