How to assign posts to categories in WordPress
Once you have the category structure created, you can add posts to categories. There are two ways you can do this. The first one is to open a particular post (or write a new one) and place a checkbox in the category you want to assign it to.
The second way is to bulk add posts to a category. To do this go to Posts -> All Posts. Next, select the posts you want to add to a category, click on the Bulk Actions tab, select Edit and finally press the Apply button.
Once you do that, you will few options you can adjust for those posts. One of them is the Categories section. Select the category you want to add those posts to and click on the Update button.
Now, the posts you’ve marked previously belong to the category you’ve chosen.