Although Cisco now markets these phones and ATAs via their Small Business line, I still think of these devices as being Linksys. Although the software that Cisco provides for these devices comes with an upgrade executable, it is only for Windows operating systems. I have found that the easiest way to upgrade a phone without using the Windows executable is to place the firmware on a web or TFTP server and go to the phone using the following URL scheme:

http://<phone hostname or ip>/admin/upgrade?<firmware URL>

For example, to upgrade a Linksys SPA2100 to the last available firmware version, I would go to the following URL:

http://192.168.2.10/admin/upgrade?https://slaptijack.com/spa2100-3-3-14.bin