I'm guessing that I am one of the few brave souls that decided to upgrade from Lollipop to Marshmallow. It was kinda a PITA so I will share with you how to pull it off. This is for devices that have been rooted or have a modified Rom. Note: 1 ----> ---> --> Once you go Marshmallow there is no going back !!! This means don't load any of the LOLLIPOP roms. I'm working on downgrading but just know....after you upgrade to Marshmallow the current RUU on the HTC website will not take you back to Lollipop. Section #1 Boot Loader --- Boot Loader --- Boot Loader Your Boot loader should be unlocked. First things first. This is for the HTC Desire 626s after it has been rooted. I'm going to assume that your boot loader is in "Unlocked" or "Re-Locked" state. I would also guess that you have installed a custom recovery like TWRP. Now is the time to use that recovery and make a Backup. After you have made the backup " To a SD Card" (remember we will be erasing everything on the phone except the SD Card)" we need to check that the boot loader is unlocked. After powering down, press Volume Down and while pressing Volume Down, press Power holding both buttons to start the device into Bootloader mode (now Download mode for One M9 and newer devices). If you have difficulty, power up normally, go to Settings->Battery and deselect Fast boot. Power down and try again. It will show the status of the boot loader. ***LOCKED*** ***UNLOCKED*** ***RELOCKED*** We need the boot loader unlocked !! If your boot loader is locked or relocked follow the instructions in the next link. http://www.htcdev.com/bootloader/ Do not use firefox browser it will not work with the HTC site correctly !!!!! Select Desire as the phone model in the list. Yes you have to register on HTC First. Sorry I didn't get super detailed this thread may become too long if I do. Section #2 Unlocking Re-locked Boot loader The Unlock_code.bin file you got from HTC Dev site needs to be copied to the same directory where fastboot.exe is located. !!! You should have fastboot.exe and Unlock_code.bin From the HTC Dev site....If not http://www.htcdev.com/bootloader/ Next on the phone go to developer options and make sure that OEM Unlocking is checked. Also make sure USB Debugging is checked. Be sure to authorize the phone to allow the computer connection when the RSA Key dialog pops up on the phone. Boot into boot loader mode again. After powering down, press Volume Down and while pressing Volume Down, press Power holding both buttons to start the device into Bootloader mode (now Download mode for One M9 and newer devices). If you have difficulty, power up normally, go to Settings->Battery and deselect Fast boot. Power down and try again. Connect phone to COMPUTER !! Then select from the menu. reboot to download mode Lol, I can be so sarcastic at times.... Sorry You need to use the fastboot command prompt. Open a dos command window in Windows and CD to the directory where fastboot.exe is located. TYPE: fastboot flash unlocktoken Unlock_code.bin hit enter and you should have a screen pop up on the phone asking you to confirm unlock of the bootloader. YAAAYYYY Your boot loader is unlocked. Proceed to the next post for Flashing Back the Stock Recovery.