Flip computer over, being sure the device is unplugged, powered off, and battery removed.
Then, locate two screws for hard drive.
Next, loosen both screws and remove plastic door.
Grasp translucent plastic tab and pull to the side so hard drive comes free of SATA connectors
Remove hard drive from computer and loosen 4 screws that hold it into it's metal housing.
Buy yourself a larger and/or faster 2.5in hard drive.
Redo the steps in reverse to reassemble the hard drive.