I've attached a PDF file with instructions on how to replace the coupler on the base of the blender. It was labeled as coming from Kitchenaid.
If you are needing to replace the coupler in the bottom of the jar, you will need to unscrew the ring at the base of the jar to remove the blade assembly.
Here are the links to the two couplers. The first is the coupler on the base, the second is the blade assembly
KitchenAid Blender Coupling - 9704230Blender Dome & Blade - 9704291