Soak the ground around the base of the post with a garden hose. When the ground becomes soft, wiggle the post to loosen its footing in the ground. When it becomes loose enough, you may be able to lift it straight out of the ground.
Dig around the base of the post with a shovel to loosen its footing. If it is set in concrete, dig around the concrete. Wiggle the post periodically to loosen the dirt around the footing and make it easier to dig. When you have removed enough dirt, you may be able to lift it out. If not, lever the post out with a length of 2-by-4-inch lumber.
Drive two 3-inch screws about halfway into the post at a point about 6 inches from the ground. The screws should be about 2 inches apart.
Place a scrap 4-by-4-inch block of wood on the ground under the screws and put the end of an 8-foot length of 2-by-4-inch lumber over the block and under the screws. Use the 2-by-4 as a lever to lift the post out of the ground by pulling down on the other end.