Reset Your Password in Windows 10 Bash / Ubuntu
If you forget your root password for Bash on Windows, you can do the following to reset your password:
First, run: lxrun /setdefaultuser root . This will open a new Bash window as root.
From there, run: passwd your_username and then enter your new root password
Then change the default user for Bash back to your user: lxrun /setdefaultuser your_username
That’s it! Enjoy Bash on Windows 10.
Alex
Doesn’t work anymore if you install Ubuntu from last Windows update.
https://docs.microsoft.com/en-us/windows/wsl/user-support#resetting-your-linux-password