To cancel already booked appointment of German embassy or Karachi consulate. Simply carefully check your appointment confirmation email.
There will be a link in confirmation email to cancel your appointment. Just click on that link and follow the procedure to cancel it. For both Islamabad embassy and Karachi consulate, appointment cancellation process is same.
Please keep in mind that, you cannot book a new appointment unless you cancel the previous one. After cancellation of your previous appointment, you can book new appointment after 2 weeks.
If you will not get your appointment confirmation email. Then please must check the junk or spam folder of your email. Sometimes it appears in junk or spam folder.