Be very careful doing that, because of the danger of damaging roots with tools. Better to lift the whole plant out of the pot, add a couple of inches of new soil in the bottom, then gently rough up the root ball with your fingers, set the plant back in the pot and sift new potting soil down the gaps in the sides. Water it well before you do this, and be sure it's all moist after you're finished, too. Then keep it in the shade for a few days after.
