Do you have to make an appointment to have our batteries replaced on our I phone six plus.

I would recommend calling them and confirming that they have batteries in Stock. If you don't ask, they will have you come in, they will do tests and say that they do not have the battery on stock, which was happened to me last week

They occasionally will take care of it if you go in store and there's a drop out on the schedule.

Yes, or waiting a long time.