VCL Delphi

procedure TForm1.Button1Click(Sender: TObject);
var
i: Integer;
begin
i := GetWindowLong(Button1.Handle, GWL_STYLE);
SetWindowLong(Button1.Handle, GWL_STYLE, i or BS_MULTILINE);
Button1.Caption := 'Line1' + #13#10 + 'Line2';
end;