Visit an Apple Retail Store - Set up an appointment with a Genius.
Visit an Apple Authorized Service Provider
Order a replacement bottom case kit online. Receive a kit that includes a new bottom case, screws, a Phillips head screwdriver, and instructions for how to remove the current bottom case and install the new one. If you feel comfortable with the self-service option, you can place your order via the web. https://supportform.apple.com/201105/