When you install WordPress, it usually comes with a sample post under the category “Uncategorized,” which is also the default category. If you want another category as the default category, you can rename “Uncategorized” to that category. However, if you already have set a default category and later decide to select another category as the default category, you can follow these steps:
- Log into your WordPress as an administrator.
- Go to the Dashboard -> Settings -> Writing. You will see the following Writing Settings.

- Under the drop-down list of the ‘Default Post Category‘, you will see all your categories. Select the category that you want to set as default.
- Click on the ‘Save Changes‘ button at the bottom to save it.
Now, when you forget to select the category of a post, WordPress will assign the default category to that post.