Ran a search for this but it's probably so basic it wasn't thought of. New owner, played with "Blink" and am ready to move on but can't close the Blink file. Tried "quit" and "close" and it shuts down the Arduino IDE. If I re-start the IDE it opens up on the Blink file. Any info appreciated.
File -> new and create a new sketch.
It will always (in theory) open the last sketch you had open, assuming a clean exit.