How change user type from contributor to author

I recommend backing up the database before making any major/bulk changes to it.

First you need to get all users with the role “contributors”, then update their roles to “author”.

$contributors = get_users(array('role'=>'contributor'));

foreach($contributors as $contributor){

    wp_update_user(array( 'ID' => $contributor->ID, 'role' => "author") );

}
echo "done updating contributors to authors";

You could have this script on an empty page template. Just load the page once, for the script to run. Then disable the script.