In order to change the data source from the ribbon in excel 2007...
Click on your pivot table in the worksheet. Go to the ribbon where it says Pivot Table Tools, Options Tab. Select the Change Data Source button. A dialog box will appear.
To get the right range and avoid an error message... select the contents of the existing field and delete it, then switch to the new data source worksheet and highlight the data area (the dialog box will stay on top of all windows). Once you've selected the new data source correctly, it will fill in the blank field (which you deleted before) in the dialog box. Click OK. Switch back to your pivot table and it should have updated with the new data from the new source.