Use this guide to replace the battery door on your Canon EOS 40D.
After removing the battery, place your index finger between the inner section of the battery door and the battery compartment as shown in the photo.
Then place your thumb slightly above the battery door hinge.
With your thumb, gently press down on the battery door and rotate the battery door clockwise. The upper hinge will pop out and then the lower hinge will pop out.
To reassemble your device, follow these instructions in reverse order.