Instructions
Turn off your Xbox 360.
Locate the HDD that you wish to replace. The HDD will be labeled with "HDD," and fits on the left, thin side of the console when positioned horizontally. Notice how it fits to the Xbox 360. This will help you install the new HDD.
Hold down the button on the HDD and pull out until it fully ejects from the console.
Align the new HDD the same way the previous one was installed. The fatter side of the HDD positions toward the back of the console while the thin side aims toward the front.
Push down on the fatter side first.
Push down on the thin side until it clicks into place.