Unfortunately, you can't just lower it. It sounds like the post hole wasn't dug deep enough and below the frost line. As the temperature drops over the winter, the ground freezes, and all the water in it expands and pushes against things like fence posts. To fix the problem, you'll need to dig out the fence post, and make the hole deeper. You can check with someone in your area if you don't know how deep the frost line is in your region; or take HALF the height of your fence (above ground), and dig a hole that deep for your fence post (So a 6 foot fence should have 3 Feet of post in the ground). You should line the base of the hole with a few inches of gravel (which keeps water away from the bottom of the post), and then add the post, ensure that its level and plumb and add your concrete. You'll need to brace the post until the concrete sets, but thats about all you need to do.