How to update status of Active Task?

When a Task is Assigned to a Group an individual User has to 'accept' the Task for it to be accepted by them. When the Task is assigned to an individual then it is 'accepted' by default. So one option might be to trigger a transition (by implemented an 'exception') from the Task that is currently assigned to to the Group to another identical Task that assigns it to the target User.

Explicitly i need to change the assignee of task by calling there task id.

Simply is there any way i can update My Task report?

I don't understand what you're trying to achieve. a Task Report will simply tell you what the current status of a Task is and who it's assigned to. What's the business requirement here?

Initial requirement was user needs to upload document for each assigned task.
Later on they also want functionality to upload multiple doc for multiple task at a time.

We have added selection to task grid so that user can select multiple task and click on it lets say bulk upload. Once user click on button we are capturing data from task report and will start new process where user can see data for all selected task. Here user can upload document for each different task.

Here we have requirement is when user click on Bulk Upload button we need to change the status of selected tasks to accepted by logged in user so that other user of that group not able to see that task.

Let me know if you have any queries.

Initial requirement was user needs to upload document for each assigned task.
Later on they also want functionality to upload multiple doc for multiple task at a time.

We have added selection to task grid so that user can select multiple task and click on it lets say bulk upload. Once user click on button we are capturing data from task report and will start new process where user can see data for all selected task. Here user can upload document for each different task.

Here we have requirement is when user click on Bulk Upload button we need to change the status of selected tasks to accepted by logged in user so that other user of that group not able to see that task.

Let me know if you have any queries.

You can't change the "status" of a task, but you could potentially manually assign the tasks in question.  Have you looked into the Task Reassignment Plug-in?