SM-G920F, HOWTO install insecure bootloader?

Hi everyone,
My question is how do you install a insecure boot loader on a FRP-locked mobile phone without loosing your user-data? I've read that this is possible to do this but haven't been able to find any guides which explains this To clarify it I only want to install a insecure bootloader which enables adb and root access on the phone without losing the stored user-data so that you can make a complete backup of the user-data partition so that you could restore deleted files such...

SM-G920F, HOWTO install insecure bootloader?

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