I've seen a lot of people ask this and I warn you to backup your WordPress database before doing any MySQL work to your WordPress.
You can run this via command line or in PHPMyAdmin:
UPDATE wp_table_posts SET post_password ='password';
UPDATE: What you want MySQL to do.
wp_table_posts: What ever you named your WordPress tables when you installed with _posts added to it since we're updating the posts.
SET: What data you want to modify.
post_password: What column in the table we are updating.
="password';: The password you're going to want for all of your posts with some syntax for MySQL to work properly.