You can consider the student table for this example. Assume that It has three columns Sid, Marks and grade 

Create table Student 
(
Sid integer primary key,
 marks integer, 
grade char(2)
);

 Now I am inserting values into this student table.

insert into Student(Sid, marks) VALUES(1,80);
insert into Student(Sid, marks) VALUES(2,90);
insert into Student(Sid, marks) VALUES(3,70);
insert into Student(Sid, marks) VALUES(4,67);
insert into Student(Sid, marks) VALUES(5,79);

The below PLSQL procedure will you the expected output.

declare
    
       noOfRows integer;
       studMarks integer;
       loopVar integer :=1;
begin
       select count(*) into noOfRows from Student;
       while loop Var <=noOfRows
       loop
               select marks into studMarks from student where sid=loopVar;
                if studMarks>=90 then
                          update student set grade ='O' where sid=loopVar;
                elseif studMarks>=80 and studMarks<=89 then
                          update student set grade = 'E' where sid=loopVar;
                elseif studMarks>=70 and studMarks<=79 then
                          update student set grade = 'A' where sid=loopVar;
                elseif studMarks>=60 and studMarks<=69 then
                          update student set grade='B' where sid=loopVar;
                else
                          update student set grade = 'C' where sid=loopVar;
                end if;
                loopVar :=loopVar+1;
                end loop;
                end;
                 /

Nice. Let us look for other approaches as well.