Originally Posted by Wordpress Permalink Settings

Optional

If you like, you may enter custom structures for your category and tag URLs. For example, using topics as your category base would make your category links like http://example.org/topics/uncategorized/.

Originally Posted by Wordpress Rockers

if you already have a running blog and you are changing the permalink, then the old URL will end at your 404 page. So here is the solution.

Install and activate WordPress Redirection plugin. Now access Redirection options using Tools > Redirection and use Add new redirection. Now type “category” in the Source URL without the quotation marks, leave all the other options unchanged, even leave the Target URL blank and hit ‘Add Redirection’ button.

That’s it. You are successfully removed the slug and redirected all the permalinks with ‘category’ to new permalinks without any slug.