The Parrot AR Drone has a total of four blades (or propellers…). Note that not all blades are equal. Each blade has a letter printed near the shaft hole. Two blades are labeled “A”, and the other two are labeled “C”.
While replacing the blades is pretty straightforward, the critical step is getting the blades in the right order. Otherwise, your drone won’t take off. (It will roll over one side.)
The image bellow1 shows the correct order.
I recommend this blade replacement kit from Amazon: Parrot AR Drone 2.0 Propellers. The kit includes 4 propellers (2 ‘A’ and 2 ‘C’), and 8 clips.
(You can find a detailed teardown of the Parrot AR Drone on iFixit.)