Remove the plant from the soil. Use something like a narrow garden trowel and push it into the soil about 3 to 4 inches away from the plant, in a circle all around the plant, and then stick it back in and pull out the soil you just separated. You should not lose any roots, or at least not enough to make a difference. Then replant the plant deeper, so the lower leaves are around one quarter inch to one half inch above the level of the soil. Keep your light(s) as low as you can without causing heat issues and have a small fan blowing on the plant and as it grows the lateral motion of the plant will cause the stem to grow thicker and stronger.