You may need to communicate with the client and ask them to send you files needed for you to complete the work. They can send them to you via Messages.
Files can be exchanged through Upwork's built-in messaging tool. But Upwork places no limitations on how clients and freelancers can deliver files to each other. Use FTP, Dropbox, Google Drive, or anything else.
As a professional freelancer, it is appropriate for you to have some file exchange methods available that you can use to receive and deliver files, and you should also be willing to work with the client's preferred method.