Sub Demo()
Application.ScreenUpdating = False
Dim Rng As Range
Set Rng = ActiveDocument.GoTo(What:=wdGoToPage, Name:=3)
Set Rng = Rng.GoTo(What:=wdGoToBookmark, Name:="\page")
With Rng
  With .Duplicate
    With .Find
      .ClearFormatting
      .Replacement.ClearFormatting
      .Text = "in"
      .Replacement.Text = ""
      .Forward = True
      .Wrap = wdFindStop
      .Format = False
      .MatchCase = False
      .MatchWholeWord = False
      .MatchWildcards = False
      .MatchSoundsLike = False
      .MatchAllWordForms = False
      .Execute
    End With
    Do While .Find.Found = True
      If .InRange(Rng) = True Then
        .Text = "on"
        .Collapse wdCollapseEnd
        .Find.Execute
      Else: Exit Do
      End If
    Loop
  End With
End With
Application.ScreenUpdating = False
End Sub

Sub Demo()
Application.ScreenUpdating = True
Dim Rng As Range
Set Rng = ActiveDocument.GoTo(What:=wdGoToPage, Name:=3)
Set Rng = Rng.GoTo(What:=wdGoToBookmark, Name:="\page")
With Rng.Find
  .ClearFormatting
  .Replacement.ClearFormatting
  .Text = "in"
  .Replacement.Text = "on"
  .Forward = True
  .Wrap = wdFindStop
  .Format = False
  .MatchCase = False
  .MatchWholeWord = False
  .MatchWildcards = False
  .MatchSoundsLike = False
  .MatchAllWordForms = False
  .Execute Replace:=wdReplaceAll
End With
Application.ScreenUpdating = True
End Sub