Step 1: Buy your Blades
Before you go to buy them, make sure you know the exact model of your car. This includes the year, make, model and further specifications such as, "type."
Go to your nearest Vianor shop and give them your car's info. They will be able to look it up and tell you your options. There will be many options for blades (you'll usually need to buy a different blade for each side). It's usually a good idea to replace both blades at once, if one has gone bad, the other can't be much better.
Step 2: Remove the Old Ones
First, pull the entire wiper assembly up, so that they remain vertical. Then turn the blade perpendicular to the arm so that the hinge is visible. You'll notice a small tab on one side of the hinge. Pull that tab* out (you'll here a snap) and then pull the wiper down, towards the car. Now that the blade and arm are separated, remove the blade through one of the holes on the blade. With the blade removed, the wiper arms are now unprotected metal, so don't leave the arm up if you turn away to get your new wipers. If they were to fall back down, it would not be good for your windshield. I like to lay down the old wiper in a way that it would catch the arm should it fall...just in case.
*If your tab appears broken, you can use a key to pop out the remaining nub.
Step 3: Attach the New Ones
Take the new wiper, and insert the arm through the appropriate hole, so that it pushes the actual wiper up. To determine which hole it goes through, adjust the hinge to be perpendicular to the wiper and line it up so that the arm's hook will go over the hinge. Once the arm is inside the hole, just line the hook over the hinge and push them together. You'll hear and feel a click when it's snapped in. Lower the arm, rinse and repeat and you're finished!
notice a big difference in their performance. I know I did.
You can throw away the old wipers, or reuse them. I personally have one in my bathroom for wiping off my mirror after showers, when it's fogged up.