you can top dress that with new soil, right up to the bottom set of leaves, the stem will just start to grow roots eventually. just make sure to let it dry out good between watering, don't get the buried stem soggy till it changes....don't get it soggy then either, but you get it
if you let that dry out pretty good, you can just unpot it carefully, add some new soil to the bottom of the pot, then fill in around the sides after you replace the plant.