How do you change the turn signal bulb on a 2004 Toyota Avensis?

If you are talking about the models that are previous of the ones with the signal bulbs implemented in the side mirrors it's an easy job. Otherwise I don't know.

So you need to push the side turn signal unit to the front of the car (push the unit from it's back side - the one to the back of the car), and after that pull it out. You can do this with a screwdriver with a covered edge. Just stick something on the edge of the screwdriver, so that you don't scratch anything in the process.