Hi,
You don’t need Proxmox Backup Server for this. Just copy your backup file to the new server into the backup folder (usually /var/lib/vz/dump/). After that, it will show up in the Backups list in Proxmox and you can restore it from there.
There isn’t an “Upload” button on the backup page — you have to move the file yourself (for example with SCP/rsync), or use the “Content → Upload” option on the storage if the file is small.