how to upadate hboot from 3.16 to 3.19 ?

[sheikharfaz]

request another unlock_code
follow the steps to unlock the bootloader again on the hcdev website http://www.htcdev.com/bootloader/

it worked thanks!!!!!

 

[sheikharfaz]

remove serialno from the getvar result in your post above

i don't know what you want
you need the latest firmware to flash a custom rom or you need to flash the latest stock rom ( by using RUU ) ???

in all cases you need to flash the latest firmware first 6.20.502.5 ( it's the latest firmware for your AT&T device variant )
6.20.502.5 is marshmallow and it's the latest official OS for this device
note :- flashing the signed firmware will wipe your internal storage so make sure to backup your files


flash firmware
1-download this signed firmware https://mega.nz/#!XV5EjYhS!gNN_rAYovaMQpaeP2OSdkfYAIEAmv6VnB-QlZ1VadMU ( rename it to something short and easy if you like )
2- open a cmd window in the folder where the firmware located
3- relock the bootloader ( if it's unlocked ) by this commands
fastboot oem lock
fastboot reboot-bootloader
3- now start flashing the firmware using this commands and replace firmwarename with the actual firmware name
fastboot oem rebootRUU
fastboot flash zip firmwarename.zip
if you see FAILED (remote: 90 hboot pre-update! please flush image again immediately)
then repeat the flashing command again
fastboot flash zip firmwarename.zip
after it complete type
fastboot reboot-bootloader


flash custom rom
now you have the latest firmware if you want to flash a custom rom unlock the bootloader and flash twrp then flash the desired rom


flash stock rom using RUU.exe
if you want the latest stock rom RUU.exe it's available on htc website here http://www.htc.com/us/support/htc-one-m8-att/news/
direst link for the RUU http://dl3.htc.com.s3.amazonaws.com...70_ATT_MR_Cingular_US_6.20.502.5_combined.exe
after flash the firmware and do all the above commands run the RUU as a normal application and follow the setup wizard

What's the difference between flashing a custom rom and ruu? And how can I install RUU?

 

[ahmed.ismael]

What's the difference between flashing a custom rom and ruu? And how can I install RUU?

custom rom is custom !! Have some feature not available in the stock rom or something else
RUU is Rom Update Utility and it's a utility to back to full stock partitions,flashing it will re write every partition to it's stock condition

About how to flash RUU
I literally write it in the post you quoted

Look at screenshot below

Sent from my HTC M8 using XDA Labs