Hello,
You could change it easily from page template option easily in the pro version of WPDM. As you posted in free version forum, I think you are using the free version of WPDM. Free version doesn’t allow to change template.
In this case you can 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 tpls>page-templates for page template or tpls>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.
.
