Cannot modify primitive's mesh

Hi All,

I have a problem modifying cube's mesh. I simply put the cube primitive on the scene, then in the script I write:

 Mesh aMesh = GetComponent<MeshFilter>().sharedMesh;
 aMesh.vertices [0] = aMesh.vertices [1] = aMesh.vertices [2] = aMesh.vertices [3] = new Vector3 (10, 0, 0);
 Debug.Log (aMesh.vertices[0].x);

these changes to cube's mesh don't affect the cube's shape and Debug.Log shows another value, not 10. The same is with following code:

 Mesh aMesh = GetComponent<MeshFilter>().mesh;
 aMesh.vertices [0] = aMesh.vertices [1] = aMesh.vertices [2] = aMesh.vertices [3] = new Vector3 (10, 0, 0);
 GetComponent<MeshFilter>().mesh = aMesh;
 

But when I create the mesh from scratch, like this

 Mesh aMesh = new Mesh ();
 aMesh.vertices = new Vector3[24] { assigning vertices here };
 aMesh.triangles = new int[] { assigning triangles here };
 aMesh.uv = new Vector2[24] { assigning UVs here }; 
 aMesh.RecalculateNormals ();
 GetComponent<MeshFilter>().mesh = aMesh;

everything works fine, the object looks according to the vertices assigned to the new mesh. Please tell me how to modify an existing mesh of a cube? I miss something really simple but it's my second day with Unity.