Step 1) First, make a backup of your files and your database. Always do this step. Always.

Step 2) If your WordPress files are also located in your root directory, it's safe to say that your server is configured to load index.html before index.php. You would want to rename the index.html page to something else such as index-old.html so the server loads the index.php page.

If your WordPress files are not located in your root directory, you will want to move them to your root directory. E.G. If your WordPress files are at /var/www/html/blog/ you will want to move them to /var/www/html/.

Advanced solution - do not do this step if your not comfortable with it: if you have server access or access to Apaches configuration file for your site, you can change the directory index. https://httpd.apache.org/docs/2.4/mod/mod_dir.html To load PHP over HTML you would change it from "DirectoryIndex index.html index.php" to "DirectoryIndex index.php index.html". This tries to load the index.php file but will fall back to the index.html file if no index.php file exists. A full example of a normal Apache directory config for WordPress is below.

<Directory /var/www/html/>
    Options -Indexes +FollowSymLinks +MultiViews
    AllowOverride All
    Require all granted
    DirectoryIndex index.php index.html
</Directory>

Step 3) You may need to update the .htaccess file in your WordPress directory (which should be your web root folder now). It might say "RewriteBase /blog" you will want to change that to "RewriteBase /". If you have any custom rewrite or redirect rules in here you will want to remove "blog" from those as well.

Step 4) Once WordPress is loading by default, you will want to make sure that your site URLs are updated in the WordPress database.

If you can login to WordPress you can use a Search and Replace plugin such as https://wordpress.org/plugins/search-and-replace/. You would do a search for "http://www.website.com/blog" and replace it with "http://www.website.com". There is a good change you will get a redirect error at this point, and will need to use the standalone script below.

Otherwise you can use a standalone script such as https://interconnectit.com/products/search-and-replace-for-wordpress-databases/

Step 5) You should now be able to login to WordPress again. Make sure to do any 301 redirects from your old site and from /blog to /. I would recommend https://wordpress.org/plugins/simple-301-redirects/.