I had the same thing happen to me... my advice is to Google this subject, I
did that and there is a very detailed step by step instruction on how to do
this. But basically what you need to do is remove the cover where the
control is and there is a bolt behind that, be careful though because if you
drop the bolt into the door good luck getting it out, then remove the torx
nut and it should come right off. Install the new one in the same way you
took the old one off in reverse order and it should be all set. its a fairly
easy fix if you are even a little handy with tools.

Yep, I've replaced both of my mirrors now.

There is a cover on the inside of the door that is just the size of the
mirror. It is held on by 3 clips at the back side and a plastic post at the
front. Pry it back gently at the rear just enough to free the cover from the
clips and then it will pop streight out.

Once the cover is removed, if the mirror is electric rfemove the wires from
the harnes. Then remove the foam padding from the door frame. Therfe is a
10mm bolt there that you can remove with a socket and extension, then pull
the mirror out from the outside of the car
Pictures here:
http://www.myfordfocus.com/how-to/mirrors.htm

Jack