I’d like to change a user’s display name using this code snippet in my theme’s functions.php file:

$user_id = 672;
$display_name= 'Les Yeux';

$user_id = wp_update_user( array( 'ID' => $user_id, 'display_name' => $display_name ) );

This is adapted from this topic in the codex: https://codex.wordpress.org/Function_Reference/wp_update_user

But I can’t get it to work. It didn’t change anything. Did I miss something?