US Visa appointment can reschedule 2 times after first appointment booking. That means totally you have 3 chances of scheduling appointment.
When you login using the link https://cgifederal.secure.force.com/scheduleappointment
In the home page you can able to see the "Reschedule Appointment" menu, click on that and reschedule your appointment.
Once you reached the maximum limit, then the "Reschedule Appointment" link will disappear in the menu
For your reference please find below picture for reschedule history.
This will applicable for both H1 and H4.