There are a few ways to thicken up corn chowder. First, you can remove a cup or so of the finished soup and either puree it or mash it, then stir it back into the soup. Second, you can mix a tablespoon of corn starch or flour with a couple tablespoons of the chowder broth or water, and then stir it into the soup. Let it simmer a few minutes to thicken up.