Do not run this on a live site until you’re tested it on a backup or local installation and confirmed valid results, i won’t take any resposibility for lost data, should that occur.

SQL Query

To run from PhpMyAdmin

UPDATE wp_posts SET post_content="your_content" WHERE post_type="post"

Inside WordPress

To use inside a plugin or your functions file (run once)

global $wpdb;
$my_update_query = $wpdb->get_result( "UPDATE $wpdb->posts SET post_content="your_content" WHERE post_type="post"" );