we also have a customization team and if you want to use them, please contact customize@wpdownloadmanager.com for quote.
Or
You need to follow the steps to customize
You need to copy the tpls/ dir from download manager plugin dir to active theme dir. (This is required to do in-case you want to update your plugin later the change wont be effected on design)
Then rename it copied tpls dir ( which is in your theme dir ) to download-manager
Now edit wpdm-all-downloads.php in download-managerdir ( which is in your active theme dir )