有这样一个json：

    [
            {
                groupname:'主要任务',
                index:0,
                tasks:[
                    {
                        id:2,
                        title:'完善通讯录aaweeeeg',
                        urgency:'正常',
                        remark:'some word here',
                        tasker_main:'henry',
                        tasker_other:['jim','john'],
                        participators:['1','2','3'],
                        publisher:'henry',
                        ddl:'1288323623006',
                        base_score:'5',
                        upvoters:['jim','小华'],
                        completed:true
                    },
                    {
                        id:3,
                        title:'task2',
                        urgency:'正常',
                        remark:'',
                        tasker_main:'12',
                        tasker_other:['小李','小王'],
                        participators:['1','2','3'],
                        publisher:'小华',
                        ddl:'1289355623006',
                        base_score:3,
                        upvoters:[],
                        completed:true
                    },
                    {
                        id:4,
                        title:'紧急会议',
                        urgency:'紧急',
                        remark:'some word here',
                        tasker_main:'henry',
                        tasker_other:['smith','harry'],
                        participators:['1','2','3'],
                        publisher:'小明',
                        ddl:'1285978923006',
                        base_score:2,
                        upvoters:['henry'],
                        completed:false
                    }
                ]
            },
            {
                groupname:'技术任务',
                index:1,
                tasks:[
                    {
                        id:5,
                        title:'task1',
                        urgency:'火急',
                        remark:'some word here',
                        tasker_main:'henry',
                        tasker_other:['jim','john'],
                        participators:['1','2','3'],
                        publisher:'mary',
                        ddl:'1285978923006',
                        base_score:'5',
                        upvoters:['jim','小华'],
                        completed:false
                    }
                ]
            }
        ]

我想通过task的id这个属性，找到特定的task对象，然后把它的completed属性改成true。
请问该如何实现这个功能啊？

PS.这个json实际上是当对象来用的，不是字符串。。我是想用JavaScript，在用户单击按钮时，把相应的task标记成已完成。

谢谢