^^^If you follow these directions you'll be able to do the job and figure out the small differences between an STi and WRX-2.5RS.
Save yourself the trouble and replace both at the same time.
To add to these instructions, buy some Dielectric grease for the two electrical connectors external to the tank to keep the water/salt/dirt out of that connection. Both of mine failed not because what's happening in the tank, but a poor connection external to the tank.
When I still had a factory warranty the dealer replaced just the main fuel level sender (fuel pump side) thinking this was causing the problem. While it did make the fuel readings somewhat more accurate, it didn't fix the issue completely. Fast forward about 2 years later same issue..
If you filled the tank, it would only register 3/4 to 7/8 of a tank initially, then slowly creep to full after about 20 mins of driving. So, I decided to revisit the snag myself. This time I de-pinned each connector (rear wiring harness side) and cleaned the each of the terminals (one by one with a small wire brush), applied some dielectric grease, and replaced both sensors. 2 years later, the fuel readings are still perfect.