Due an upgrade on iphone contract?

I have an iphone 4s on 24 month contract with o2 but i'm able to upgrade today. I got the phone at phones4u, where do i go to upgrade? Because phones4u don't do the phone i want on o2 but o2 & carphone warehouse do & it is cheaper in carphone warehouse.

You have to understand that O2 contract from Carphone warehouse you have mentioned may be for new customers only and you can't upgrade to the same deal if you are already with O2.

it is almost always better to switch phone operator on upgrade unless your current one gives you the same phone/deal you want.

Speak to Carphone warehouse, if they don't give you that deal, then most likely you have to switch to another operator which will consider you as a new customer. Make sure you get good network coverage from that operator where you live

Go with Carphone Warehouse.
I got my Note II for Free on 24 Months with unlimited calls and Texts with 1Gb Data for £27 when O2 wanted £99 Upfront with £32 per month for 600 minutes, unlimited text and 500Mb Data.

Phones4U and Carphone warehouse
Are just agents for O2 (or any network)

The contract is always with O2.
The agent just gets their fee… For the sale.

Once your O2 contract is available for upgrade (or early / buyout upgrade)
you can go anywhere. O2, or any phone shop.

You can even collect the agents fee yourself,
if you sign up with Quidco or TopCashBack and click through those links to O2
and then upgrade on line (through your online O2 account - you do have one, yes?)

eg upto 150 pounds back on a new contract
Quidco - sign up (free) here
http://www.quidco.com/user/1983737/o2-mobiles/

TopCashBAck - sign up (free) here
http://www.topcashback.co.uk/ref/notneeded