Short answer
HubSpot's KB: only you can change your own login email; Super Admins and HubSpot Support can't change it directly. One email address can be the login for only one HubSpot user. With SCIM on, change it in your identity provider. If you only sign in with Google, reset your password first.
1. The Security tab
Preferences in my demo portal, email blurred.

2. Leaving a company
A departing user's records stay; an admin removes or reassigns the user.