Spray a little oil-based product, which you can purchase from your local home construction store, onto the joint between the top and bottom of the metal post. You also can use standard vegetable oil.
Allow the oil to set in for a few minutes.
Twist gently the top of the post from the bottom. You may want to put on rubber gloves to get a better grip on the two parts of the post and to protect your hands from oily residue.
Gently tap the top of the post from the bottom with a rubber mallet. This is especially useful if your metal post is rusted.