How to update data in pivot table with laravel

First of all, I'm using laravel 5.1. Now, the scenario is; I have an exam page where they can click a button to take the exam, the action of the button is to attach the exam_id and user_id to the pivot table called examattempt_user. Now that we have a record there, I want to save the grade of a user after the user submitted the exam (clicking the button), and save/update the grade into the pivot table where the exam_id and user_id was saved. Here's my table:

Pivot table Fields:
id
exam_id
user_id
grade

via Chebli Mohamed