The Applications built into the macOS is pretty decent software. There are third party versions for each of these Applications that my be more powerful, but many people get by just fine with the builtin Apps from Apple. One of those pieces of software is the Calendar App. I have used third party calendar Apps like BusyCal for years, but the macOS Calendar App is more than adequate for many users. If you are using the Calendar App, then read this article which provides information about one of the most important features of any calendar App, editing appointments.