I'm not sure on this one, but my best guess would be to press up and down buttons on the trip computer when it says "press up and down for options". I would say the option would be in there.
Also good to know you can turn them off because that's one reason I hesitated on a Calais I was looking at. Reversing next to a car or through a shed door it beeps as though there is something right behind you and makes you double question your judgement which gets annoying.
There is a pale blue wire with a plug on it that comes from the right hand rear tail light harness, that is the wire for your reversing harness. For those of you that want to shut it up, it's as simple as disconnecting that plug. I turned the missus one off because previous owner didn't put the correct bumper on with the sensors after a repair so it would just have one continual beep while in reverse.
Mine just gives me a continual beep as soon as I put it in reverse, and it's shitting me to tears! Figure I may as well just disconnect it, but easier said than done.
What happened to the good old days where in a VH Commodore you only had 6 different coloured wires on the whole car! Just in the boot alone I think my VY has more colours than the rainbow.