Changing account email address

[Smilo]

My O2 app is showing an email address I do not recognise (it’s an O2 one) when I go to change it to the one I want to use it tells me that email address is already in use. How do I get this fixed

[MI5 · Accepted solution]

@Smilo 

You'll need to call customer service.
Numbers here Guide: Coronavirus Community Help and Support 
When it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
8.00am is the best time to call if you can.