how create zip file from update.app file?

[Pimpin Kabrayat]

Any body can help me how to repack update.app (stock rom) into flashable Zip
My phone have unlocked bootloader and have twrp.
Impossible to flash stockroom with update.app file via twrp.
I`ve tired using custom rom so many bug.
I want rolling back into stockrom with keep twrp n unlocked bootloader.
I've lost my backup
Sorry bad English
Device SCL-L21

 

[TheWarrior12345]

1. extract boot.img and system.img for update.app using huawei update extractor
2. copy them into fastboot folder on your computer
3. go into fastboot mode and type this on you computer:
fastboot flash boot boot.img
&
fastboot flash system system.img
3. when completed just type "fastboot reboot"
4. wipe data and cache in recovery to start your phone like new

that`s what i do when i flash stock rom