Comment on 2016 WordPress SEO Tutorial Permalinks by SEO Dave.
Changing the post/page slug changes the URL, hence the 404 errors and currently WordPress doesn’t have a built in feature to automatically 301 redirect changes to post/page slugs.
If this is a one of slug change (not going to change dozens of post slugs) a single line in your .htaccess fill will fix the 404 error and conserve any link benefit (most of it anyway) sent to the old URL.
See the 301 Redirects Tutorial for full details, specifically “A Simple 301 Redirect” example:
Redirect 301 /old-seo.htm https://stallion-theme.co.uk/new-seo.php
Add the above to your .htaccess file changing the two URL parts, the first part is the old filename location (entire URL of the old post minus the domain name), second part the full URL of the post as it is after you changed the slug.
If you plan to make a lot of changes consider a redirect plugin, there’s plenty of free WordPress redirect plugins. Where I’ve moved a significant number of posts from one site to another I’ll use a redirect plugin rather than the .htaccess file solution.