Replacing the battery of an HTC EXCA100 is a simple process, requiring no tools.
Power the phone off. Remove the back cover of the Phone, exposing the battery.
Grasp the battery by the two indents on either side of it, near the bottom. Lift up, pulling the battery loose. Remove the battery completely.
Take the new battery and orient it correctly -- the arrow should be facing towards the camera lens, face up.
Insert the battery at an angle, pushing the battery in until it is aligned on the top edge. Push the rest of the battery in, snapping it into place.
Reattach the battery cover, and power up your phone.