Exercise 1.3-  Go back to the original program. Now add a second LED to the circuit.   Program the LED’s to alternate blinks i.e. when LED 1 turns on, LED 2 turns off, then when LED 2 turns on, LED 1 turns off.

I added another Led into the D3 input and then rewrote code 1, where I made them high and low alternatingly, with a small delay in between