It's not hard to do but is not obvious. The arms are curled back at the ends and the blades push back out of them. You will need to lift the arms away from the windscreen and as jug notes there may be clip that you have to push in for it to release. It's easier to let the blade pivot so the arm goes through the slot in the blade. You just push the replacement in to place, you can see a rounded end on the fitting that goes into the arm. I just did it last week with new Bosch blades on my car before it's MoT, I change them every other year as it's possible to fail with ratty blades. You did used to be able to buy the insert by the yard for change at Motor Factors, nor sure if you still can but that way is easier to do than you would imagine.