Using your thumbs, slide the lower case away from you.
The lower case may stick in the front on a tab just above the center of the optical drive. Be careful as you work the case off, or you may break the thin strip of plastic just above the drive.
Insert the new latch by rotating the clip up and pushing the latch into place. Then rotate the clip back down, pushing in at the area where the clip bends around the latch. Make sure to insert the clip fingers back into their slots.