Off the top of my head....
Remove the back cover, remove the battery, unscrew the small screws and the 2 sections will come apart (Be careful because the button for the key flip is under tension so it might pop out at you!!) I think I twisted the two sections to release the tension, once the 2 sections are apart there's a grey coloured plate inside, slide this back and this will release the rubber buttons. Installation is the reverse process. You will need to put the spring for the key flip in and rotate the 2 sections (about 3 times) this creates the tension for the flip.