If your account login was done through your Facebook or Google account, your password is the same as the one you use to log in to those services. You can reset your password directly through Facebook or Google.
If you have a Khan Academy account and forgot your password, visit this link.
Once you enter your email, a unique URL will be sent to that inbox. Once you access that link, you will be prompted to set a new password. If you do not receive the email, check your spam folder.
If you are logged into Khan Academy already, visit your Settings page.
Remember to click Change password to save the changes!