iphone backup restore using itunes. It asks for a password, then I changed password, but still itunes says the pasword is not correct

You can't change the password for an existing encrypted backup. You set the password when you first enabled encrypted backups in iTunes. If you can not remember the password, you will not be able to restore that backup at all.

There is no back door or workaround for this. That's the entire point of encrypting the backup. It can't be used without the correct password.