Have just changed my wingmirrors (to put red ones on a white car) and have the following tips:

To remove wingmirrors (gently push bottom of mirror in, put flathead screwdriver down back of mirror between black plastic brackets and very gently twist screwdriver to pop bracket and release mirror glass, you can then release each of the clips easily and remove the existing cover, the new one then just pops on)

Replacement wingmirrors are £29 each (discounted) at a Fiat dealer but on ebay there are various sellers retailing FIAT OE ones for £10.50 each in a primer finish or a bit more pre painted. Its around £40-£50 to get a pair painted

BH