You can let it cook for a little longer (uncovered). This will let the extra moisture escape. Also, you can add cornstarch to thicken your soup. To do so, stir together 1 tbsp of cornstarch with about 2 tablespoons of cold water in a mug. Then, pour the cornstarch slurry into the soup while it simmers. Stir well to combine, the soup should thicken within 1-2 minutes. Repeat if needed.