Do you mean post_content
?
add_action('acf/save_post', 'your_function_name_here');
function your_function_name_here($post_id) {
$post = get_post($post_id);
$content = get_field('your_field_name_here');
if ($content) {
$post->post_content = $content;
// remove this filter to prevent infinite loop
remove_filer('acf/save_post', 'your_function_name_here');
wp_update_post($post);
}
}