Shut off main house water valve.
Drain water from the shower pipes by turning handles on; turn handles to off position when all water has drained.
Place towel on shower or tub floor to protect the finish.
Pop the valve cap off with the ice pick. Remove the handle screw with the flat-head screwdriver. Take off the shower handle. Remove any other parts between the valve and the handle.
Remove any flanges or mounting nuts with a crescent wrench.
Remove the backside wall access panel or the front mounted shower panel to the interior wall shower plumbing.
Close any stop-check valves by turning them to the right.
Remove the lock nut from the valve body with a crescent wrench.
Insert a towel around the manifold underside to catch any water drips.
Unscrew the threaded manifold unit or cut copper pipes to the unit, if soldered.
Install the new manifold unit per product instructions; if soldering an unthreaded unit, remove the cartridge and replace after soldering the manifold in place. Crimp copper pipes for unthreaded manifold installation with the channel wrench.
Reassemble the shower handles, attaching to the new manifold in the reverse order from removing them.
Reattach access paneling. Repair any access holes made to reach interior wall plumbing.