Hello @bijdageraad,
Greetings and welcome.
You can change it easily from the page template option in the pro version of WPDM.
For the free version, you have to modify the relevant template.
In this case, you can follow the below documentation,
Template Files
By default, you are using link-template-default.php for the link template and page-template-default.php for the page template.
Now copy the files you want to modify from /src/{ComponentDir}/views dir to /{active-theme}/download-manager/ dir.
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.
.
