Step 1: Switch the default user to root
Start the command prompt in Windows and enter:
ubuntu2004 config --default-user root
This has switched the default user of my ubuntu20.04 to root.
The first symbol of the command may be different in different WSL versions. The difference is as follows:
Step 2: Reset user password
Type in WSL
passwd username
This username is my own username. I wrote the username here for invisibility
and then received instructions to enter the password and confirm the password.
Enter the password and confirm it
Step 3: Switch back to normal user
Do it again like step 1, except that the user is no longer root but his own username
ubuntu2004 config --default-user username