This guide will take you through the step by step instructions necessary to perform this procedure.
Before you proceed, please note that you will need the iPhone with the original back casing when you take it to Apple for support. So remember to replace the iPhone with the original casing under such circumstances as you would end up losing the warranty on your iPhone. For the process, you will need a #00 Phillips screwdriver. You can buy this from any hardware store in your neighborhood or purchase one from eBay.
Step 1: The iPhone 4 back panel is primarily secured through two screws on either side of the charging port at the bottom face of the iPhone. Use the screwdriver to unsecure these screws from their position
Step 2: Hold the iPhone with the touchscreen display facing downward and the charging port on your end. Now slide the back casing up by applying gentle pressure with both hands on the panel.
Step 3: The casing should budge. You may now carefully remove the panel.
Step 4: Place the replacement casing over the back side of the iPhone and place it in the slid-up position (as shown in the image below):
Step 5: Now hold the casing as in Step 2 and gently slide the replacement casing into position. You will hear a 'click' sound when done correctly.
Step 6: Secure the two screws back into position using the Phillips #00 screwdriver. Ensure that you do not secure them too tightly.
The process is complete. You can now enjoy your new-look iPhone 4.
You can also checkout the video guide to see how its done:
Source : Iphonehacks