It’s always a good idea to make theme modifications by creating a Child Theme for the theme you would like to customize. Probably the best reason to do that, would be because you will lose any modifications that you make when the theme is updated in the future – and that happens quite a bit. So.. that being said, I’ll let you read the info and decide what might be best for your situation.
On line number 349 in the Twenty Twelve functions.php file you will find a line that begins like this:
<?php comment_reply_link( array_merge( $args, array( 'reply_text' => __( 'Reply', 'twentytwelve' ),
The word ‘Reply’ – ( ‘Reply‘, ‘twentytwelve’ ) – is what you will need to edit. Change it to what you like, and save the changes to the file. Be careful to change only the word Reply before you save the change.
I’m sure there may be a way to just add a filter to the functions file, but I think that’s what you’re looking for, and this seems the most direct route to it.
Don’t forget to take a backup before you edit, just in case.
