WordPress allows you two fairly simple and easy method to transfer posts from one user to another user.
Find the below two simple methods:
1: Deleting The First User’s Profile Account
Simple method to transfer posts from one user to another is by simply deleting the first user’s profile account. During the account deletion process you will be asked whether you want to keep all items associated with that profile. And most important is, two which exiting user account want to link theses all posts and its comments to.
2: Updating SQL Database
You can also transfer post from one user to another user executing SQL command, using this method there, you do not need to delete the existing user profile account, you can run the SQL statement either using phpMyadmin and hit the associated database or alternatively you can search WordPress plugins that will allow you to directly execute SQL statements from you wp-admin page.
Now open the user account page under wp-admin and now note both user ID.
Now make a SQL query and execute your SQL statement, make sure user ID is correct before executing below command.
Note :- Please take a SQL database backup and keep it secure place before doing anything for safety in case of any data lose.
mysql> UPDATE wp_posts SET post_author=NEW_AUTHOR_ID WHERE post_author=OLD_AUTHOR_ID;