On the S13 SR20DET, you can't take the Upper Intake Manifold off without loosening the lower/brackets. On the S14 SR20DET, the easiest way to replace the knock sensor is as follows:
Removing the upper intake:
-Remove the throttle cable (no need to remove the TB).
-Remove the hoses from the back (closest to driver) side of the upper intake
-Remove a section of intake piping from the TB (DONT remove any of the million vacuum hoses from the TB/Intake on the front side, you don't need to)
-Now, remove the 4 bolts and 2 nuts that hold the Upper intake on.
-Next, remove the brackets that hold the Upper intake the the block.
-You should now be able to pull the upper intake off of the lower intake and flop it towards the front of the car. If you do it right, you don't have to remove any of the vacuum hoses from the front of the intake/TB
Removing the knock sensor:
-First locate the knock sensor. You can follow the wiring harness down and feel the bolt head. It is directly behind the 2nd (from the rear) tube in the lower intake.
-This is the fun part, with a 12mm wrench, reach your left hand behind the back of the lower intake. You should have enough room to get your arm back there and get the wrench on the bolt. I was able to break it loose by pushing really hard on the end with my left hand, and pushing from the top with my right hand with a large 3/4" open ended wrench pushing down on the 12mm wrench.
-If you are lucky enough to be able to break it loose, just unscrew the bolt. This is the only bolt that holds the sensor on and you should now be able to pull the sensor out.
-Getting it back in is less tricky, just get the bolt started and tighten it as best you can. Often people overtighten this bolt and damage the knock sensor, and this is nearly impossible to do from this horrible angle
Reassemble!