How to turn my power steeing switch off?

The power steering on my Citroen c3 has stopped working again.Last year I took it to 2 garages ,could'nt find a fault.1st garage turned it off,then a wk later took it to the 2nd garage they could'nt find anything wrong and turned it back on and it came back on.I was hoping to try turning it off then back on but I don't no where to start looking to turn it off .Any ideas?