{
This example uses a button to copy the value of a field in
the previous record into the corresponding field in the
current record.
}
procedure TForm1.Button1Click(Sender: TObject);
var
   SavePlace: TBookmark;
   PrevValue: Variant;
begin
   with Customers do
   begin
    { get a bookmark so that we can return to the same record }
    SavePlace := GetBookmark;
    try
      { move to prior record}
      FindPrior; 
      { get the value }
      PrevValue := Fields[1].Value;
      {Move back to the bookmark
      this may not be the next record anymore 
      if something else is changing the dataset asynchronously }
      GotoBookmark(SavePlace);
      { Set the value }
      Edit;
      Fields[1].Value := PrevValue;
      { Free the bookmark }
    finally
      FreeBookmark(SavePlace);
    end;
  end;
end;

{
To ensure that the button is disabled when there is no
previous record, the OnDataChange event of the DataSource
detects when the user moves to the beginning of file (BOF
property becomes true), and disables the button.  Detection
occurs on scrolling and editing, not selection with the mouse.
} 
procedure TForm1.DS2DataChange(Sender: TObject; Field: TField);
begin
  if Customers.Bof then
    Button1.Enabled := False
  else
    Button1.Enabled := True;
end;