How to "Enable Super User Access" for users with Partner Community license

To update the existing users, go to the the User's Contact page (Manage External User -> Enable Super User Access)

Alternatively, or if you have to do this in large batches, you can do it via the API.

1. Open Developer Console
2. Select Debug->Open Execute Anonymous Window

3. enter a short piece of code like the below:

   User u =new User(Id='xxxxxxxxxxxxxxx',IsPrmSuperUser = true); update u;

If you want any new portal user to automatically become a superuser, you could implement a trigger on User. Make sure the method in which you do the update has @future context. Example of the method to call in your trigger:

@future
private static void updateUsers(Set<Id> users){
    List<User> usersToUpdate = new List<User>();

    for(Id userId : users){
        User u = new User(Id=userId);
        u.IsPrmSuperUser = true;
        usersToUpdate.add(u);
    }
    update usersToUpdate;
}