Compare two tables sql server 2012 : If I have two tables ( Grades , Students mark)

grade                                      
StudentId    Mark    examId 
1              10     1
2              9      2
3              15     1
4              26     3

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StudentMark
                                   
StudentId    Mark     examid
1              10      1
2              5       2
3              15      1
4              8       3
And I want to compare the data between this two table , and if there different in mark I want to replace the grade(mark) with StudentMark(Mark) using the bulk 

How to do that ?
