You can retype the captions, or use a macro, similar to the following:
'===============================
Sub ResetCaptions()
'retrieve original field names
'if captions have been typed into pt
Dim pt As PivotTable
Dim pf As PivotField
Dim pi As PivotItem
Set pt = ActiveSheet.PivotTables(1)
For Each pf In pt.VisibleFields
For Each pi In pf.PivotItems
pi.Caption = pi.SourceName
Next pi
Next pf
pt.RefreshTable
End Sub
'============================