How can I edit before I complete creating an appointment without going all the way back to the beginning?

You either have to go back, or book the appointment, and then edit the appointment. Here are the steps for editing an appointment after it has been booked: From your Schedule screen,

Tap on the desired appointment (this will open up the appointment card)
On the appointment card, you can edit the following:
Day/Time
Duration
Services provided
Products used
Notes