A brand-new WordPress installation includes a new post category called “Uncategorized”. It is a default blog post category. When you write a new blog post and forget to assign it to a certain category, WordPress will automatically include the blog post to the “Uncategorized” category. The “Uncategorized” category itself cannot be deleted unless you have set a new default category. The question is, how to change the default post category in WordPress?
In WordPress, category is one of the elements to sort blog posts, other than tag. Technically, category and tag are called taxonomies. You can create as many categories as you need. To add a new category, you can go to Posts -> Categories on your WordPress dashboard. From here, you can also edit and delete the existing categories. There is one category you can’t delete: the default category. You can’t delete the default category as category is one of the required WordPress components. You need to have at least one category on your WordPress site.
How to Change the Default Category in WordPress
To change default post category in WordPress, first, go to Settings -> Writing on your WordPress dashboard. Simply select a category you want to set as the default category on the Default Post Category dropdown. Make sure to click the Save Changes button to apply the change.

The Bottom Line
Category is one of the WordPress default taxonomies used to sort blog posts. Every WordPress website has a default category. Every blog post you write will be included to the default category unless you set it yourself. Category — as well as tag — are used to sort blog posts in archives. Every WordPress theme has a default template file (archive.php in most cases) that controls the layout of the archive pages. If you are willing to change the layout of the archive pages, you can simply edit the template file. Or, if you have no PHP knowledge, you can use a theme builder plugin like Elementor and Divi Builder to create a custom template for the archive pages (e.g., the category archive page) in a visual way.