Mesh mesh = GetComponent<MeshFilter>().mesh;
mesh.Clear();
mesh.vertices = new Vector3[] {new Vector3(0, 0, 0), new Vector3(0, 1, 0), new Vector3(1, 1, 0)};
mesh.uv = new Vector2[] {new Vector2(0, 0), new Vector2(0, 1), new Vector2(1, 1)};
mesh.triangles = new int[] {0, 1, 2};