Add / Remove user role programmatically in Drupal 8

 For this we need to load a user first

<?php

use Drupal\user\Entity\User;

// pass the correct user id here.

User:load(4); 

or you can load current user

<?php

use Drupal\user\Entity\User;

User::load(\Drupal::currentUser()->id());

Then we add a user role 

// pass machine name of the user.

$user->addRole('administrator');

$user->save();

To remove the role we can use this code

// pass machine name of the user.

$user->removeRole('administrator');

$user->save();