My car is a 2004 Impreza sedan. This procedure should be identical for all 2000 - 2007 models. Other Subarus will be similar but not identical.
Step 1: Remove the "Combination Meter Assembly" (aka Instrument Panel, or Gauges)
Start by moving the steering wheel as far down as possible, to give yourself space to work.
Take off the black trim piece around the instrument cluster by removing 2 screws in the top of the piece. Sorry my picture is cut off. See step 2 for a better view.
There are also 2 snaps at the bottom of the trim piece, pull it directly towards you firmly until they "click" out. To get the piece to fit past the steering wheel as you remove it, try tilting toward you.
Step 2: Remove More Screws, Then Disconnect Wires
Reach behind and unplug 3 green connectors.
Then fully remove the assembly. Keep the clear cover on.
Step 3: Replace the Bulbs
Most of the light bulbs are for things like the turn signals or check engine light. Don't replace these unless they are broken. (to check, make sure they illuminate briefly when you first start the car).
Only 5 actually illuminate the gauges and needles. You should replace all 5 at once so the color stays the same. See the attached PDF, they are #1, 2, 5, 10, 11.
Replacement bulbs are readily available online, search for "#74" (this refers to their size). I recommend LEDs, because they will last longer than the car. For color go with natural white, NOT cool white or other colors. Other colors will not show through correctly because the gauges have green and red color already.
As you can see in the picture, there are black, blue, and red bases, plus bulbs with big and small heads. Don't worry about this. The bases are all interchangeable and you can use the same replacement bulbs in place of both originals.
Step 4: Put It All Back Together and Test
If one or more of the bulbs aren't lighting up, note their location, then open everything back up. Turn the bulb the opposite way in the socket. This is because many LEDs are polarized - they only work in one direction.