Update custom UM field from within php

I’d like to update some Ultimate Member custom fields for specific user IDs from within some of my custom scripts. Most examples I find seem to default to the ‘existing’ or ‘current’ user. Pseudo code below as to what I’m trying to do. Help?

    
um_fetch_user("3848");
$to_update[] = array('name_of_field' => 'its_value');
um_update_profile($to_update);  // which um function to use? 

//  do we need this too?  
//  update_user_meta("3848", 'name_of_field','its_value');