How do I activate device details

[Judette]

I have a PAYG sim and for the past few months I have been getting messages asking to upgrade my phone since it is a very basic one and only supports up to 3G which will soon be turned off. I went to the trouble of buying a new 4G phone and transferred my sim. Imagine my surprise today to still get a message showing that I need to upgrade my phone. On further investigation it appears that my account seems to have the 3G recorded. I saw some instructions to log into O2 and click on device details to update them and then do something with the phone itself. Trouble is, on logging into my O2 account, I can't find device details anywhere. Does anyone know where I should navigate to to update? 

[Enlli]

Not sure you can in the resigned My O2 (Web or App). Theoretically it should pick it up from the IMEI number registed on the network, but that takes time.

The text messages are auto generated, just ignore.