Edit in JSFiddle

function scoreGrade(){
    var score = prompt('점수를 입력하세요!','');
    var grade;
    
    if(score<60){
        grade='F';
    }else if(score>=60 && score<70){
        grade='D';
    }else if(score>=70 && score<80){
        grade='C';
    }else if(score>=80 && score<90){
        grade='B';
    }else {
        grade='A';
    }
    
    document.getElementById('score').innerHTML = '점수: ' + score + '<br>';
    document.getElementById('grade').innerHTML = '등급: ' + grade + '<br>';
    
    
}