If you got a blank page when you clicked 'upload'/'download' icon from Files function in your control panel, that is because you did not install Java runtime in your local. You can download and install it to your local first from this link, and then please follow this instructions to enable it. After that, you can use the function freely.
1. Login to your hosting account control panel --> Files
2. Select the target files/folders and click Download icon on that page to download the site contents to your local. Click Upload icon to upload your local site contents.