User access system based on roles and in some cases we want to assign role based on specific business requirements. 

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 we 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();