Depends when your contract is up. Have a look here.
I did it on o2 when mine finished, will probably be worth filling in the form whatever seeing as with unofficial routes you have the inconvenience of having to wait for updates, as well as the risk of it bricking up.
If you can't do that I'd suggest going full whack and jailbreaking it yourself. You can find plenty of instructions on Google.
If your out of contract on the 3GS, just call the network and they do it for free!!!
o2 even have an online page for requesting it. There is no reason why they'd say no. Jailbreaking won't actually unlock the carrier just so you know. However there is a program you can download free through Cydia that will.
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I did it on o2 when mine finished, will probably be worth filling in the form whatever seeing as with unofficial routes you have the inconvenience of having to wait for updates, as well as the risk of it bricking up.
If you can't do that I'd suggest going full whack and jailbreaking it yourself. You can find plenty of instructions on Google.
That said, she's not the most savvy so ideally I need it to have all working features once it's been done.
http://www.iclarified.com/entry/index.php?enid=4253
edit: if your phone isnt jailbroken google 'limera1n'
o2 even have an online page for requesting it. There is no reason why they'd say no. Jailbreaking won't actually unlock the carrier just so you know. However there is a program you can download free through Cydia that will.