polygons or nurbs? Or subDs?

If polygons, select the faces, Edit Polygons > Extract. Then you can re-orient it. To reattach, select hand and arm and Polygons > Combine. Snap and merge the points together at the seperation.

For nurbs, you select an isoparm at the wrist, Edit Nurbs > Detach. Re-orient your hand. You'll need to move the seam of the hand to match that of the arm. Select the isoparm you want to become the seam, Edit Nurbs > Move Seam.

Select the isoparm at the end of the wrist, Shift select the isoparm at the end of the arm. Edit Nurbs > Attach.

You may need to go into the attach options to adjust the attaching methods.