The easiest solution is to reduce the soup by simmering it without the lid to evaporate some of the liquid, but this can take some time and could concentrate the seasoning too much. Alternatively, thicken it with cornflour, or arrowroot (as you do for a sauce). If the soup contains cooked vegetables, all or some of these can be pureed (by mashing, sieving or blending) and then stirred back into the soup to thicken it. Alternatively, add some finely chopped vegetables which will cook quickly in the soup and can also be pureed.
** Asian Recipes