Removing Mirror Caps
How to Remove Wing Mirror Caps
The mirror caps "clip off". They can be removed by inserting a plastic trim tool in the gap running along the top of the wing mirror and then push the painted part away, in the opposite direction from the glass.
If you don't have any plastic trim tools fold the mirror in, use a screwdriver under the edge closest to the car and closest to the floor and gently prise it off, or pull carefully to try & avoid snapping the brackets that hold it on. Also you much be careful around the indicator part of the wing mirror as this can break easily.
With thanks to Kenti for the picture