How to copy between domain and subdomain?

Website URL

dev.yamauchifamily.com

Error Message

No error displayed

Other Information

Hi all, can someone send along a link or instructions as to how I can copy files from my domain to a subdomain? Essentially, I want to develop out of the subdomain and copy the files to the domain when they are ready to publish. I’m just not sure how to do this as File Manager limits copies to the root of the domain or subdomain but not inclusive. I tried FTP but I can only see the root of the domain so I’m not quite sure how to copy the files between the two. Any advice is appreciated. Thanks.