You can log out and log back in as a different user.

If you mean changing after posting, I change post authors with a small SQL statement using the post id and user's id. If I want to switch a post (99) to myself and my user id is 3, I'd issue:

UPDATE evo_posts SET post_creator_user_ID = 3 WHERE post_ID = 99;

If you have access to an SQL client you can run a statement like this against your database.

i'll use log out method,i need it to be very simple method,thanks