We'll create fresh WordPress site with Simple SEO Pack installed. You have 20 minutes to test the plugin after that site we'll be deleted.
With Simple SEO you can set your custom content for the “keywords” and “description” meta tags in each post or page using the standard WordPress metabox interface.
You can also set default values for each meta tag which will be displayed in all your pages, although this is not a SEO best practice.
In order to update your database you can run the following queries using your favorite MySQL admin tool, changing your table prefix if needed (a backup is strongly recomended).
UPDATE wp_postmeta SET meta_key = '_sseo_meta_keywords' WHERE meta_key = 'meta_keywords';
UPDATE wp_postmeta SET meta_key = '_sseo_meta_description' WHERE meta_key = 'meta_description';