For those who setup Ubuntu Linux Bash on Windows 10 but forgot what password they used. I set this up quite a while ago on a computer that I seldom use, and I needed to reset my password. Here are the steps I found from Ask Ubuntu.
bashand make a note of your Linux username (this need not match your Windows username)
In Windows admin command prompt (Super+X, A) change the default user to root:
lxrun /setdefaultuser root
Now Bash on Ubuntu on Windows logs you in as root without asking password
passwdcommand in Bash to change the user password:
Change the default user back to your normal user in Windows command prompt
lxrun /setdefaultuser your_username