I don't think you can anymore, I broke my phone earlier this year and tried to buy out so I could upgrade. They said they did not do it anymore. I had to wait 2 months.
I had previously done it which annoyed me a little.
I'm on 02 & my phone has broken and I now want a new one but my contract doesn't run out until 24th Jan. I called them up and asked if I could upgrade early and they said no, not until 24th Dec, but I
CAN buy out the last 2 months of my contract for a fee of £20. My line rental is £25 per month, so that is a pretty good offer! The person I spoke to gave me a number to call, but I don't have that to hand at the mo unfortunately.