Not possible. Remember the password or create a new certificate.
If you know the password, you can actually change it in the Edit->Preferences->Signatures->Identities&Trusted Certificates->More dialog, but it will ask you for the current password before letting you to enter a new password. This functionality is useful if you think that your password was compromised and you want to change it. As BA said it is impossible to change a password if you forgot it.