Stand a ladder up against the post. Climb up the ladder with a hand saw.
Stop climbing when your shoulders are even with the top of the beam.
Lay the saw flat on top of the beam, perpendicular to the post.
Saw into the post as far as you can comfortably. Use the beam as a guide to keep the saw flat and horizontal.
Climb down off the ladder. Turn the ladder to face the other direction. Climb back up the ladder and cut through the post from the other side to meet the previous cut line.