Hi,
There is no option to reset the number of downloads of an order.
What you can do is to temporarily increase the download number limit setting of that file in particular in the settings of the corresponding product. That way, it will only apply to this file in particular, and not the whole shop.
Then, once he downloads the file, put back the download number limit setting of that file to what it was before.
Now, it's also possible to reset the number of downloads of an order, but it requires going in your phpmyadmin, and it's not easy to find find what needs to be changed if you're not familiar with phpmyadmin and the table structure of HikaShop.