Not hard at all. I've done it three times (here in the states, DO NOT get a rebuilt unit from NAPA).

Anyway, secrets are:
1) Remove fan assembly first.
2) Release tension on belt, then take out the bolts holding the alternator in.
3) Wrestle the alternator out of the mount and slowly rotate it so you can get to the back, disconnect wiring, and pull alternator out.
4) Before putting the new one in, using pliers, rotate and pull on the shims in the mounting holes to move them out a bit, so the new alternator drops right in.

That last one is the most important. It will be hard to get the alternator to let go when pulling it out, by moving the shims out of the way (by just a few millimeters), the new one will go in easier than the old one came out.