I'd use a dynamic range name.
See Debra Dalgeish's site for some nice tips:
http://contextures.com/xlNames01.html#Dynamic
But you can right click on the pivottable and show the wizard. Hit back until
you get to the dialog where you specify the range, then make your change and
finish up.
(The dynamic range name may make it so you never have to do anything but hit the
refresh button, though.)