Remove the cover on the back of the stem by extracting the bolt indicated below:
Lift the cap off to expose the hardware underneath. It lifts straight up. Now you can see the upper and lower pivot retaining screws. Remove the upper one all the way and loosen the lower one a little bit.
Now use a phillips screw driver or 5mm allen key to push the upper pivot out of the stem.
Be careful when pulling the spring out as there are upper and lower spring perches that will come out at the same time. These parts are not adhered to the spring and may fall to the floor. Do not lose these, they are important.
Install the new spring making sure a lower and upper spring perch are installed. The lower is the flat one.
Close the upper link of the stem, being careful not to pinch yourself. It will take some pressure to line up the holes so that you can push the pivot through.
Once the pivot is through and lined up flush to both sides, install the upper set screw and then tighten the lower set screw.
Install the cap and cap retaining bolt.