Back in April Microsoft released details of a new API do do bulk import of user profile properties in SharePoint Online. Say you have a field with an employee id, and you want to set this for all users in your organization. As Azure AD does not allow you to map custom fields into the user profiles automatically you have to set the data your self. Previously you could use the user profile API as an admin to do this, but it was not possible if you were for example using an automated app with app tokens, and you also had to loop over all profiles, updating one at a time.
You can read more about the API and see a full code sample at https://dev.office.com/blogs/introducing-bulk-upa-custom-profile-properties-update-api.