When the Allow Changing Email Address option has been selected in Admin > Settings, users will be able to edit their own email address/login at any time.
To change your email address go to My Account to update your profile.
If the Allow Changing Email Address option has not been enabled, then non-admin users are not able to change the email address/username set against their account for accessing the site.
Admin users will need to make these changes on their behalf.
To change a user's email address, click on their name in Admin > Users, enter the new details, and then Save to update their profile. The user will need to login to the platform using the updated details moving forward.
*Note that a user's password will remain the same even if the email address is changed - they always have the option to reset it themselves from the login page.