Basically, you will have to remove the complete instrument panel from the car. Some cars it comes out as an assembly, some cars a piece at at time. I've never done an Impreza so I'm not sure about it. Usually, a good rule of thumb is that if you have a pass side airbag, the whole instrument panel will have to come out as an assembly, after taking off a few trim pieces to access various
fasteners. It doesnt have to come all the way out, just back far enough to get the HVAC assembly out. You may (almost definantly will) have to discharge the a/c as well, as the evaperator coil will come out in the same assembly. This is not a small undertaking. It is illegal to vent the refrigerant to the atmosphere, even R134a, the new stuff. It is also quite expensive, 10-30 bucks per pound, depending on where you live.
If you are unsure of what you are doing, seriously consider having a professional do it for you, or it could turn into a real nightmare. Good luck.