请问 如何操作json中的某个元素？？

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