Basically do what alaricljs said, gently pull on the cap until you feel it slip off the stem.
Then, what you want to do is gently push it to one side, while simultaneously lifting the opposite site. Do all this very gently, but be firm. Eventually the side that you're pulling up will pop off, and you will see the small white inserts.
Apply some lube (I prefer White Lithium Grease) to the slots on those inserts, then re-install, basically by reversing the steps you took to take the key off.