So you're first setting the value of Nmum to 0.5 for row C, then you want to renumber all rows starting from 1?
If so, here's one way to do this in SQL:
create table t (
name varchar2(1), num number
);
insert into t values ('A', 1);
insert into t values ('B', 2);
insert into t values ('C', 3);
insert into t values ('D', 4);
insert into t values ('E', 5);
update t set num = 0.5
where name = 'C';
update t
set num = (
select n from (
select name,
row_number() over (order by num) n
from t
) s
where s.name = t.name
);
select * from t
order by num;
N NUM
- ----------
C 1
A 2
B 3
D 4
E 5