Hi Emotech,
I did some testing - I’m pretty sure you cannot do exactly what you are trying. The e-mail address belongs to Facebook, not to Auth0, so you are unable to change it (it would require Auth0 making an API call to Facebook to change the e-mail address, which is a very bad thing).
Depending on your use case, you should be able to find a way to do what you want. You have both user and app metadata for your facebook users, where you can store additional info. This is not completely ideal, as storing the e-mail here would make it possible to get out of sync with the facebook e-mail. But it is a possibility.
Take a look at your use case, make sure you really want to change the e-mail (knowing that you cannot change the e-mail address of the user’s facebook account), and if so, you can use the metadata.
John