At each side of the wiper blade fixing you will see a plastic grip (marked with raised plastic spots) that moves up and down. You need to hold these grips with your forefinger and thumb and force them down towards the windscreen and wiper rubber. This will allow you to pull the wiper blade off the wiper arm.
The wiper arm has a hole in it, and the wiper blade has a protruding plug that fits into this hole. By forcing the plastic grips down to the windscreen you are removing the plug from the wiper blade hole, allowing the blade to be pulled off the arm.
David, November 2013