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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