Assign a category to all articles

If you can access the database directly, you could

1. Check for the ID of the category you want to assign
2. Check for the Author ID

Then run the following MySql (on your own risk):

INSERT INTO `#__term_relationships` ( object_id, term_taxonomy_id )  
SELECT ID, 1 FROM `#__posts` WHERE `post_type` = 'post' // where 1 is your desired category ID

UPDATE `#__posts` SET `post_author` = 2 // desired author ID