There is no need to reconfigure Ubuntu, but, if you have more than one distribution installed and Ubuntu is not your default, you must also specify the distribution in addition to the root user.
Option 1 -- Ubuntu is the default WSL distribution:
wsl.exe -u root
Option 2 -- Ubuntu is not the default WSL distribution:
First determine the distribution name:
wsl.exe -l -v
Then use the distribution name with -d/--distribution:
wsl.exe -d <distro> -u root
Then go about your business to reset your regular user account. Stealing the one-liner from @NotTheDr01ds, assuming your distribution name is "Ubuntu":
wsl.exe -d Ubuntu -u root passwd username
Typical Ubuntu distribution names include:
Ubuntu: When installed from the Microsoft Store or wsl --install as the "unversioned" app.
Ubuntu-18.04, Ubuntu-20.04, or Ubuntu-22.04: When installed from the Microsoft Store or wsl --install as a specific version of Ubuntu.