I personally would do this using CSS
// css
.target.your.label {
text-transform: uppercase;
}
You can do it in PHP by using strtoupper()
<?php
$fields = get_field_objects();
if( $fields )
{
foreach( $fields as $field_name => $field )
{
echo '<div>';
echo '<h3>' . strtoupper($field['label']) . '</h3>';
echo $field['value'];
echo '</div>';
}
}
?>