Sharon, you can edit both PIN and PIN2, see Options\
Change PIN and Options\
Change PIN2 (see the attached image).
If
PIN is blocked, you can use
Unlock card (you'll have to provide the
PUK); and the same can be done with
PIN2.
The difference between
PIN and
PIN2 is the following:
the first code protects the address book, the SMS archive and the other common settings - this is the PIN everybody calls "PIN".
PIN2 is not often used and many people are not even aware of its existence. It is used to protect other data on the card, such as the prices for phone-calls of different types. Most often mobile operators do not issue the PIN2 to the customer.
SIM Manager allows you to edit both these codes.
As for
PUK - it is the
unlock key you need to provide when you wish to unblock PIN (and PUK2 for PIN2). This key is kept secret because if things were different, one who doesn't know the PIN could enter it several times in order to block the card, and then enter the known PUK to gain access to the data. In other words, if PUK is disclosed, the PIN is useless.
The PUK is usually printed on a sheet of paper that comes with the SIM card itself. Also, theoretically, the customer must be the only person who knows the PUK; if a copy of it is kept at the operator then that would mean that my privacy is compromised (anybody who is allied with an "insider" could steal my card and then use it for malicious purposes).