If upgrading to a refresh tariff you can get an early upgrade or if on a sim only and wish to take a full phone contract, otherwise you need to pay off what you owe on the current contract. Speak to o2 on 202 to have your options fully explained as we cannot see your account on here so don’t know your circumstances.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.