The OS EPROM chips both carry white stickers that indicate which OS version they have "burned" into them. In order to see the OS EPROMs, one must remove the ribbon cable that connects to the main board and runs to the keyboard.
Using a good IC-puller tool, grip one of the EPROMs tightly. With care, pull upward to remove the EPROM. The chip may need a bit of wiggling from side-to-side in order to get it free from the socket, but be careful not to wriggle too forcefully and bend the pins.
Repeat the process for the other EPROM chip.
When the EPROM chips are free, replace them with the EPROMs that have the desired OS version. When inserting the EPROM chips into the sockets, be careful that the pins align with the socket holes well, or the pins may bend as they are pushed into the socket.
OS version 4.10 is the last release from Ensoniq.