Not sure, i've been trying to figure it out but can't seem to find the info anywhere. I assume it will be something like holding down a number of buttons at the same time to make the switch. I guess your best bet would be to call a dealership and ask or check your manual.

I did this on my 1999 996, you need to rotate one of the knobs between the clocks for 5 secs I think. Its described in the owners manual.