Use the file upload to add files to the /tftpboot directory.
Backup: 'Backup' button compresses /tftpboot/ to /root/backup/tftp.bak.tgz; after that it presents the backup for download. Restore: If the backup file does not exist in /root/backup/tftp.bak.tgz then the 'Restore' button will be hidden.
To 'Restore' a previously downloaded backup, use Diagnostics -> Command Prompt -> File to upload to browse to the file and then click on upload.
After that, backup will be ready to be restored.