Yes you can.
You need to pay 6 x your monthly tariff price.
If you call customer services they will talk you through it.
Some more help here
https://community.o2.co.uk/t5/Pay-Monthly-and-Pay-Go/Cancelling-Your-Contract-A-Guide/m-p/566774/hig...
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.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)