Update set of tuples with new values : Oracle

declare

rCount integer;
j integer;
cursor cur_my_cursor is select * from my_table_name;
tRecordSet cur_my_cursor%Rowtype;
begin
rCount := 0; j:=1;
select count(id) into rCount from my_table_name;
dbms_output.put_line(rCount ' Records to be updated');
open cur_my_cursor; for j in 1..rCount
loop
fetch cur_my_cursor into tRecordSet;
update my_table_name mtn set mtn.unique_key = j
where mtn.field_one = tRecordSet.field_one
and mtn.field_two = tRecordSet.field_two
and mtn.field_three = tRecordSet.field_three
and mtn.field_four = tRecordSet.field_four
and mtn.field_five = tRecordSet.five;
dbms_output.put_line('Record id ' tRecordSet.field_one ' has been associated with an unique key of ' j );
end loop;
close cur_my_cursor; end;