How can I change my mobile phone number in my EU Login account if I do not have access to this phone number anymore?

You can manage your mobile phone number through the corresponding menu in your account: Manage my mobile phone numbers. Please note that this menu will require you to authenticate with a second factor.

In case your mobile phone number was the only second factor configured with your account, or if you do not have access to any other second factor, you will have to use the emergency procedure described onn the page An application requires a second factor authentication, but I do not have access to it anymore

This procedure will remove all your second factors from your account and should be used as last resort.