You can do it like this without overwriting the core code:
add_filter( 'acf/get_valid_field', 'change_input_labels');
function change_input_labels($field) {
if($field['name'] == '_post_title') {
$field['label'] = 'Custom Title';
}
if($field['name'] == '_post_content') {
$field['label'] = 'Custom Content Title';
}
return $field;
}
This code goes into your functions.php file for example.
Since i think not too many people needs this function, i don’t think we need a GUI for this, so we can mark this feature request as solved.