Polycom 550s aren't exactly "old". They can run older software, though.
You can download the new loads from Polycom's website directly. The phone can download them from a TFTP server, FTP server, HTTP server or HTTPS server. Use something simple like Starwind TFTP server or the like. TFTP is generally the easiest way to flash these things.
Provisioning, though, is another story, and I'd recommend reading through the administration guide for it.