1.

var btn = new Ext.Button({text:'ddd'});
btn.on('',function(b){
    b.addClass('newCSS');
});

2.

/*编辑*/
.edit {
background-image: url(../images/toolbarIcon/edit.png) !important;
}

/*发布*/
.release {
background-image: url(../images/toolbarIcon/release.png) !important;
}

var btn= new Ext.ButtonGroup({
 
 id : 'edit',
       text : '编辑',
       handler : onRelease,
       iconCls : 'edit'//样式图片
 });

function onRelease(){
  btn.setIconClass('release');//更改图片
  btn.setText('发布');
}