There's a very nice regulated PSU board inside. Near the rear end where the AC from the back is connected, there's a jumper wire on a plug.
The plug has 3 holes. The socket has 4 pins. In 120v mode, the plug is on the 3 rearmost pins leaving the frontmost pin exposed. To change to 240v, move the plug toward the front of the unit 1 position onto the 3 frontmost pins so the rearmost pin is exposed.
These were the kind of jobs I used to love when I was a "qualified tech". Nothing better than a 1-hour minimum bench charge for 5 minutes work.
It takes longer to explain how to do it than to actually do it…