With the mirror off you will find 2 lower clips (circled below)...
...and 2 upper clips (also circled below)
The above 2 pictures were taken with the cap removed so you can see the clips.
Using a sunny spot (or a torch) and a thin flat blade screwdriver, go in to each hole and lift the
tab over each of the clips. Start with the lower 2 and gently pull the bottom of the cap outwards.
Then release the top clips and lift the cap up.
The cap will now come clear of the wing mirror housing.
You can now clearly see the 4 clips and tabs.
To refit the cap, insert the top 2 tabs first and push the cap down. Once they have
engaged you can push the bottom of the cap in until the lower 2 tabs click in place.
Options
Smart now sell these in almost all of the standard smart colours. They are coloured
all the way through, not painted so they aren't affected so badly with stone chips.
Smart also do chrome covers as part of the chrome kit. These just stick on over the top of
the existing caps. They look shit in my opinion but some of you weirdos like that sort of thing.