WSL (Windows Subsystem for Linux) mounts C: as /mnt/c inside the WSL system.
Therefore, you can access your Windows user’s home directory using
cd /mnt/c/Users/<username>
In order to find out what the correct <username> is (it’s not always your normal username, especially if you logged in with your Microsoft account or changed your username), run
ls /mnt/c/Users
Note that you will get a permission denied error when trying to access /mnt/c/Users/Default User and /mnt/c/Users/All Users. This is totally normal and usually does not matter.
Original source: Windows blog article