Have you created an id or ID?
id should be in a small alphabet.
Never create database table column names in the uppercase alphabet.
All column names should be in the lowercase alphabet.
Like this -
id, rollno , subjects, marks, class , section, result, etc.
Change this line
$result=mysqli_query($mysqli,"SELECT * FROM marks WHERE RollNo=$RollNo");
to
$result=mysqli_query($mysqli,"SELECT * FROM marks WHERE id=$id");
Select data by id and update by id.