How can I edit post data before it is saved?

The wp_insert_post_data filter can do that:

add_filter( 'wp_insert_post_data' , 'filter_post_data' , '99', 2 );

function filter_post_data( $data , $postarr ) {
    // Change post title
    $data['post_title'] .= '_suffix';
    return $data;
}