Change a button label and add a space?

My client would like me to change the label of a button and add a space beneath it. Have a look at this image:

Clicky click.

This image shows the "New" button that appears in the shopping cart where a user can add a new billing address. It's produced from generated code somewhere and I need to change the label of the button so that it reads "Edit" instead of "New". Then I need to a space beneath it. (<bk>)

Can anyone tell me where the source file is where I can edit this?

Thanks in advance. I appreciate it.