on an update page, it should be populating the email field with the value for the record being updated, so the email field should not be left blank.
In security assist, when a user logs in, the User ID is stored in a session. Open the bindings panel and expand the session collection, there should be a binding named SecurityAssist_UserID (assuming UserID is the name of the ID column of the users table, if your ID column has a different name, the session will be named to reflect that)
Open the server behaviors panel, and double click the server validation server behavior to edit it.
select the Unique database value server validation and edit it. Fort the Key Value settings, click the lightning bolt icon and select the User ID session binding.