How can I modify the existing tag or add a new tag

I am unable to find any code in which I can modify existing tags or add a new tag according to my own choice.

$bag = new MetadataBag();
$bag->add(new Metadata(new ObjectName(), new Mono('Pretty cool subject')));
$writer->write('1.eps', $bag);

This code assigns a random new tag. 

I want to modify existing tag. Please help?