Start on the driver's side of your A4. Most blades are held in place with a tiny clip. Push that clip up towards the arm and push the blade backwards, as if you were sliding it down the wiper arm.
Disconnect the old blade
Once the blade is free from the hook, push the blade a few inches down the arm to disconnect it. When the hook is fully visible, the blade will be free and can be taken off.
Position the new blade
Position the new blade so that the the hook is pointing at the wiper attachment. The hook will go over the top of the attachment.
Lock the blade in place
Pull the blade toward the end of the arm. You'll hear a click when it locks into position.
You're all finished with the driver's side. Repeat the process on the passenger side of your A4.