I ran the following commands from the Windows command prompt to reset my password. Anything after the # is a comment:

ubuntu config --default-user root
ubuntu.exe # (to run WSL in the current Windows command prompt)
whoami # (to see who I'm currently logged in as)
tail -3 /etc/passwd # (to see the last three users added to the Ubuntu system)
passwd user01 # (to change the password for user01)
exit # (to exit WSL)
ubuntu config --default-user user01 # (to set the default user for the WSL terminal)

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Substitute debian.exe as appropriate