The Haynes manual says,press in the sides of the cover to release the internal clips and withdraw the cover from the mirror body. I have tried this rather vague instruction but with no success.Any body got a more detailed approach?
ford fiesta mk8 2012 - wing mirror cover removal - elekie&a/c doctor
It is only clipped on but you may have to be a little more forceful .Try pouring hot water over it or use a hair dryer or hot air gun to fan some heat over the cover to soften it.