[ZE551ML] Firmware not updating to latest version, ie, 2.20.40.59.

Hello

My device is running on 2.19.40.22 & it is rooted.
I am trying to update the firmware to latest update which is 2.20.40.59.
I tried 2 methods, viz:
1. Copying update file in internal memory & restarting the device.
2. Using ADB sideload.

Result
While trying to update from internal memory, it shows installing system update for some time & than shows "Error" with a dead android bot(same which is displayed when entering recovery mode).
While updating using sideload method, it transfers the file successfully to the device & when it tries to install, it says file is not proper.
Please help someone.

Thanks