Move All Articles to /Blog directory

I have not tested this but I think this should work in theory:

1. Export all posts in root domain in Tools -> Export
2. Install and activate WordPress Importer in your /blog/ subdirectory
3. Import all posts to this subdirectory using the importer
4. Clean up (if needed)
5. Done.

The above assumes you only want to move posts and not the entire root domain. I suggest to try the above steps in a dev site and not in a production site (for testing purposes).

That you @codex-m. I have tried that when readers visit my articles InfoguideNigeria.com through Google and other search engines, it will return to Error 404 page.

Please how can I fix this?

Well if you are sure the posts have been moved correctly and accessible at the new location. You simply need to 301 redirect the old post URLs to the new location (/blog/). This way, your visitors will be redirected to the new URL.

Please read this guide on doing 301 redirects: https://www.wpbeginner.com/beginners-guide/beginners-guide-to-creating-redirects-in-wordpress/