If there is really one trunk then you can't break it apart further. However, most basil plants sold at the grocery store are just a bunch of small ones grown closely together. To break it up, just prepare a bigger pot, with potting soil, and then remove the basil from its store-bought container and split it apart with your hands. You want to create a few even groups to go in the new, bigger pot. Ideally, there would be one plant per group, but you don't want to break them up too much and risk tearing the roots too much. I just did four quarters and it's worked fine. I have a huge basil thing now and I did it in a long, skinny rectangular pot.