Shut the main water valve in your home.
Remove the handle from your Kohler shower, using an Allen wrench to loosen the nut on the bottom of the handle.
Remove the steel clip holding the brass stem in place. The clip is what helps hold the stem in place.
Extract the stem from the mixing valve with a pair off pliers, turning counterclockwise.
Push the new stem into the Kohler brass mixing valve. You'll set the new stem into the same spot as the one you removed.
Reinsert the clip that holds the stem in place.
Set the handle back on the stem and the nut on the bottom of the faucet.