How to update only starting 5 rows???

Hi,
 
I have a table in SQL with a column Name(varchar(50)) only. There are 100 records . There are names like as A, B & C. Suppose there are 30 Name like A, 30 Name like B and 40 are like C.
Names are like as in my sql table:
a
a
a
a 
a
a
a
a
b
b
c
a
b
a
c
c
a
b
.....
Then I want to update only 5 starting rows(which contain Name A) as B.