@Thomas-12
You just remove any auto top ups that you have setup and stop using the sim.
It will expire after 6 months.
If you want to move your number to another network, get your PAC from O2 and give it to your new provider.
Guide: How to get your PAC [June 2021 Update]
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.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.