jquery中怎么修改css样式

<div id="Btndiv" onclick="changeCss()"></div>

        <script>

            $(document).ready(function (){

                //第一种

//                $("div").css("width","100px");

//                $("div").css("height","100px");

//                $("div").css("background","cyan");

                //第二种

//                $("div").css({

//                    width:"100px",height:"100px",background:"red"

//                });

                //第三种

                $("div").addClass("div1");

                $("div").click(function (){

//                    $(this).addClass("div2");

//                  $(this).removeClass("div1");

                    $(this).toggleClass("div2");

                });

            });

    </script>