Make sure the phone is turned off by holding the red End Call/Power button down until the phone shuts off.
Close the phone and find the battery cover button, located on the backside of the phone underneath the hinge.
Using your finger, press down on the button to release the back cover. Then, lift the cover and remove it from the phone completely.
Remove the battery by pushing down on it from the top while gently pulling it outwards.
Insert the replacement battery by gently pushing the bottom of the battery into the battery recess. Then, push the top in until the battery retaining tab clicks over it, holding it in place.
Place the back cover back on the phone by inserting the bottom first followed by the top, and press down gently until it clicks into place.