Editing author of a node Drupal 6
The node author is usually editable in the 'Authoring Information' tab/section of the node-edit page. If that data isn't available for you (may have been removed by another module or other customization).

If you're comfortable editing the data in the database (via phpMyAdmin or another tool), have a look at the node table. You will see that each row contains a value in the uid column - this refers to the user id (uid) of the authoring user. User ids can be matched up to the corresponding users in the users table. Simply update the uid value in the node table for the appropriate nodes (likely ones with uid = 0) and you should be good to go.