Here is some code for you. I am assuming you have only 1 table in your sheet
and that you drop down is in G1. Also you will need to change the page field
name from "This" to whatever you have...

Private Sub Worksheet_Change(ByVal Target As Range)
   Dim pvt As PivotTable
   Dim pvi As PivotField
   
   If Target.Address <> "$G$1" Then Exit Sub 'Validation list in G1
   
   Set pvt = Target.Parent.PivotTables(1) 'Change the table?
   Set pvi = pvt.PageFields.Item("This")  'Change Page Filed Name
   
   pvi.CurrentPage = Target.Value

End Sub

This code will go directly in the sheet. Right click the sheet tab select
view code...

Not sure that Jim addressed your question......

I think what you want is to choose a data fieldfrom a validated list of data
fields and then have ypur pivot pick up your choice and change its data
field. If so, the code you need is as follows:
(right click your sheet tab and click on view code - paste the following
into the Worksheet_change event)

Dim pvt As PivotTable
   Dim pvtf As PivotField
   Const VALIDATED_CELL_NAME As String = "DataNames"
   If Target.Address = Range(VALIDATED_CELL_NAME).Address Then
       Application.ScreenUpdating = False
       Set pvt = Target.Parent.PivotTables(1)
       Set pvtf = pvt.DataFields(1)
       If pvtf.SourceName <> Target.Value Then
           pvt.PivotFields(Target.Value).Orientation = xlDataField
           pvtf.Orientation = xlHidden
       End If
       Application.ScreenUpdating = True
   End If
   Set pvtf = Nothing
   Set pvt = Nothing

All you need to do then is to name your validation cell (I have used
'DataNames') and then copy this name into the constant definition in the code
instead of 'DataNames'