Apologies upfront for what may be a regular question. But I'm new to apple and indeed iphones.

I have a 3GS which I've jail-broken and unlocked using Redsn0w so that I can use it on Vodafone. I'm currently at version 3.0 with a modem firmware of 4.26.08. everything is working fine....But I would like if possible to upgrade to 3.1.2. Been reading a few blogs and I am a little confused by it all. I just want to know what is the best way of upgrading to the latest version of the software and not loose jailbreak/unlock privileges.

Thanks