Your registration "appointment" isn't really an appointment at all. Instead this is the start of your registration window — the earliest date and time you may select your classes. It is not a meeting; rather, you simply log in to myGateway when you're ready and pick your classes for the semester. This is completed 100% online at your convenience.
For example, if your email says you can register on November 29, 2021, at 10:30 a.m., this means you may start registering on this date and time up until the day before the start of spring classes. There's no need to "reschedule" if you can't do it right then, simply log in when you're ready.