Turn on the 'Show user profiles' toggle under the 'User Profiles' section. By doing so, a child form for the user table becomes available for you to set it up. When you open a portal, a new icon will appear beside the logout button for the users to access the child form to update the user record.
If you would like your users to update some records other than their own profile, these records have to be linked to the user, so they can be shown on the user's profile page. Which then would allow the users to update them.
On the legacy site app.miniExtensions.com
To allow portal users to edit their profiles, you should enable the Profile Page feature. You can find it by going to: