Your email address is the one you sign in with, and the one notifications go to.
Changing it
Click your name at the bottom of the sidebar and choose Settings. The settings pages are listed down the left: Account, Profile, Security settings, Email preferences, Accessibility, Agent preferences, Integrations, Classroom Sync and Billing.
Open Account, edit the Email field and click Save. You will need to enter your current password to confirm the change.
If your school signs in with Google or Microsoft, your email is managed there and cannot be changed here.