Locate the silver button on the back of the phone, near the bottom.

Press down the button with one hand whilst sliding the back cover of the phone away from the antenna.

The back cover should slide off easily.

Insert a plastic spudger near one corner of the battery (preferably one furthest from the phone's antenna).

With a firm grip, gently lift the end of your tool up. The battery should easily raise out of its place.

Now that the battery is exposed, use your fingers to remove the it from the phone.

Note the metal connections on the phone, which are now exposed with the battery removed.

There are corresponding metal connectors on the replacement battery.

When you install the battery, be sure both connectors are matched up. Otherwise, your battery will be unable to supply power to your phone.