Same problem here. The metal scroll wheel suggests a machined part alle made out of metal, but the scroll wheel axle is in fact made of plastic, less than a millimeter in diameter on the spot it broke. This construction is almost certain to fail within two years under normal workload. The torque forces of the motion sensor click will be enough to get it to fail over time.
Howver, disassembly is not too difficult since only the back part of the top cover (opposite of the buttons) is attached to the body of the mouse, not the buttons. themselves So if you use both hands to get your nails in the crease between the two parts near the edges on the back side (opposed to buttons) and press on the cover with both your thumbs, the top pops off. Then uncrew the four torx (T-5H size) screws of the casing. Then unscrew the three screws of the printed circuit board to be able to retrieve the scroll wheel.
The broken hexagonal part of the axle is most likely still sticking in the socket of the wheel motion sensor (some sticky lubrication grease is involved). This part is so tiny that is virtually impossible to glue in on the scroll wheel axle.
Perhaps the only way to repair this is to file off the remainder of the axle end down to the little flange, creating a proper surface to glue a small hexagonal piece of a tool set on to it.
Or fit a suitable scroll wheel from a unused (wired) mouse. I don't recommend buying the same type of mouse as replacement; this one should be regarded as "consumable".