How to upload custom buttons to theme via FTP client

Hi! Loving the theme :) I have a quick question regarding uploading your own buttons to the theme. The big pink button in the header (mine says "listen right away") is not really fitting to the rest of the theme, so I have decided to design some custom buttons to put on the site via Filezilla. Question is, where do I find these files so that I can replace them with mine? Best regards, Magnus