WSL manages the default login user from DefaultUID registry.
To edit this registry value, open Registry Editor or regedit.exe from start menu
Go to this registry path,
Double click on the DefaultUID value and change it to ZERO. Zero is for root user and 1000 (Decimal) or 3e8 (in hexadecimal) for normal users.
Open wsl.exe in command prompt. The prompt changes from $ to # (means root user). Run passwd command in Kali, change root password as usual.
Now go back to previous registry key, change Zero to previous value (or 3e8 in hex).