Purchased my car certified preowned and the previous owner had all 18 radio presets setup. Is there a way for me to reset these to default prior to setting my own?

So are you asking how to clear the stations currently stored? I mean generally speaking someone at the original dealership may have preset a couple of stations in order to properly demo the system's audio capabilities, but other than that, new cars don't come with stations pre-programmed at the factory. Think about it, your vehicle was assembled in ‎Canton, Mississippi‎, so they wouldn't be programming it for say Seattle, Washington or Bangor, Maine or wherever it's final destination might be.

Just go ahead and program the radio with your favorite stations and call it a day.

Get and read an owner manual.

Or find the correct radio fuse and remove it to wipe all memory.