How do I change my email address? OIt won’t let me in the settings.
You are probably referring to the phone app? There, you cannot, but you can in the web app (https://www.xertonline.com/account/settings)
The link you provided is the one that won’t let me update my email address.
You likely are trying to change it to another email that is already registered.
That would make sense as I’d set up an account for running some time in the past.