Hi,
you can try and follow the link below documentation,
Template Files
by default, you are using link-template-default.php for link template and page-template-default.php for the page template. As documentation, you have to copy the file in the theme directory naming a folder called download-manager, then paste it in page-templates for page template or link-templates for link template. And you have to replace [download_link] with <a href="[download_url]" class="btn btn-primary" target="_blank">[link_label]</a> in related PHP file.
Thanks.