see fiddle
Edit fiddle - JSFiddle - Code Playground
The slider changes only one of the values in the geometries attributes.position.array
This value is at position 3, see the code in the gui section at line 49.
This relates to the x coordinate of the second THREE.Vector3 that I added when I created the tetrahedron.
You need to figure which values in your geometries attributes.postion.array are the correct ones that relate to the x,y,z that you want to change.