// updating the bulk records in custom object
list<Student__c> stnewlist=new list<Student__c>();
list<Student__c> stlist=[select id, name,COURSE_FEE__c,library__c from student__c where Country__c='India'];
for(Student__c st:stlist)
{
    system.debug('student record is'+st);
    st.COURSE_FEE__c= st.COURSE_FEE__c - st.COURSE_FEE__c+0.1;
    st.E_mail_id__c='bulkrecords@bulk.com';
    st.States__c='Andhra Pradesh';
    st.library__c='a029000000JwWDt';
    st.City__c='Vijayawada';
    stnewlist.add(st);
}
update stnewlist;

st.COURSE_FEE__c= st.COURSE_FEE__c - st.COURSE_FEE__c+0.1;

if (st.COURSE_FEE__c != null) {
    st.COURSE_FEE__c= st.COURSE_FEE__c - st.COURSE_FEE__c+0.1;
} else {
   // Handle null case
}

List<student__c> stlist = new List<student__c>();
stlist = [select id, name,COURSE_FEE__c,library__c from student__c where Country__c='India']

List<student__c> stnewlist = new List<student__c>();

//Check if List is not empty before proceeding to the business logic

if(!stList.IsEmpty()) {

}

// updating the bulk records in custom object
List<Student__c> stnewlist=new List<Student__c>();
list<Student__c> stlist= new list<Student__c>();
stlist = [select id, name, COURSE_FEE__c, library__c from student__c where Country__c='India'];
for(Student__c st : stlist)
{
    system.debug('student record is'+st);
    if( st.COURSE_FEE__c!= null)
        st.COURSE_FEE__c= st.COURSE_FEE__c - st.COURSE_FEE__c+0.1;
    st.E_mail_id__c='bulkrecords@bulk.com';
    st.States__c='Andhra Pradesh';
    st.library__c='a029000000JwWDt';
    st.City__c='Vijayawada';
    stnewlist.add(st);
}
update stnewlis;

/ updating the bulk records in custom object
List< student__c > stlist=[select id, name,COURSE_FEE__c,library__c from student__c where Country__c='India'];
for(Student__c st:stlist)
{
    system.debug('student record is'+st);
    st.COURSE_FEE__c= st.COURSE_FEE__c - st.COURSE_FEE__c+0.1;
    st.E_mail_id__c='bulkrecords@bulk.com';
    st.States__c='Andhra Pradesh';
    st.library__c='a029000000JwWDt';
    st.City__c='Vijayawada';
    // stnewlist.add(st);                    // not required //
}
//update stnewlist;      // not required, use existing list which is already changed and just update it.
update stlist;