How do I stop my Arduino from executing all code, and it WON'T save?

Development Tools IDE 1.x
1

Hello.
I am new to Arduino and started a project on my Mac.
I started a simple LED project, and X-ed out the Arduino window.
After I started a new project on the same board, it kept running the same project.
I now am unable to access serial ports or upload any project, because it gives me an error message stating:

Arduino: 1.8.12 (Mac OS X), Board: "Arduino Uno"

Error opening serial port '/dev/cu.usbserial-1420'. (Port busy)

How can I quit my beginning "LED" project to start a new one?

Thanks!