I removed the housing (unscrew the 4 screws on the back) and found the reset button inside. I believe you can instead also just unscrew the single screw in the 1/4"‑20 UNC mounting hole on the bottom and use a thin needle to press the button. I guess, this is equivalent to the “Registration Pin” on the Femto Mega and you could probably follow the instructions at Femto Mega Firmware Upgrade Instructions - ORBBEC - 3D Vision for a 3D World.
Anyway, I got into the “upgrade mode” by pressing the button while powering on the device and then used USBDownloadTool.exe on Windows, as described at Bolt firmaware udpate failed 2 - #14 by zhonghong, to finally upgrade the firmware.
I could then use OrbbecViewer and the ROS 2 node to get images from the device.