insert or delete

Hi, i have this table libro_prestito that contains:
id_libro|id_autore
--------+---------
s1      |        5
n3      |        1
s2      |        2
k1      |        1
n2      |        2
n2    |     3
n2    |     4
n1      |        1
n1      |        4

now i must to update the id_libro column, but with this update I can have
the necessity to delete or insert some new element. My question is : is
better to delete the old reference and then insert the new or what ?
Ok, my english is very very bad. A little example: I must to change
id_libro = 'n2' in 'n3' and change/update old item and (ok, this the
maximum case, but i need one code for all) delete the item
"n2      |        4" and add the item "n2      |        5".


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