FYI users are maintained via Xero Practice Manager. To change the email associated with an FYI user, this needs to be done in Xero Practice Manager. Refer to New, Changed and Deleted Users from Xero Practice Manager in Managing Users.
If your practice uses Xero Tax, not Xero Practice Manager, users are maintained via Xero Tax. This is from the Xero Tax menu Business > Settings > Staff menu and clicking into the staff member's details.
If you want to change the email address of an FYI user, this must be done in Xero Practice Manager, or Xero Tax. The change will then sync across to FYI.
Note: that the synchronisation across from Xero Practice Manager or Xero Tax happens overnight. If you need the change to take affect immediately in FYI, you can do a manual synchronise. Refer to Synchronising Clients, Staff, Jobs, Time, Tax Returns with Xero Practice Manager.