you can do it on the net. sign in or create yourself a sign in at the O2 website (its called my O2) and you can change you tariff. I did the same thing myself!
It could be worth checking that you are not getting text messages that you are being charged for. My Girlfriend was getting messages from an advertising company that cost her £1.50 per text she was receiving. If you are getting these send a reply to that text with the word stop.
be careful doing that, if you didn't authorise them to send. Some of the texts you receive are sent randomly and replying just lets them know they have sent to a "live" number