You can reset your password if you have forgotten it and are not logged in by going to Profile->Forgot Password
in the menu.
Enter your email address (the one you used to to register or place an ad).
Click the [Send] button and a password reset will be sent to your email inbox.
Open up the email in your email client and click the link Reset Your Password in the email.
Enter your new password twice in the password boxes, click the [Reset] button.
Then login with your email and new password.
If you wish to change your email/login
You can change your email address by logging in with the email address you registered with. If you have forgotten your password, first follow the procedure above. This is
assuming you still have access to the email you originally registered