updateUserTeam.php

/**
    * Update the Drupal user with new team id
    */
    public function updateUserTeam($username, $team_id) {
        
        //Load user
        $user = \Drupal::entityTypeManager()->getStorage('user')->loadByProperties([
            'name' => $username,
        ]);
        
        $user = reset($user);
        
        // If user object has loaded...
        if(!empty($user)) {
            $loadeduser = \Drupal\user\Entity\User::load($user->id());
            
            $loadeduser->field_team->target_id = $team_id; // Change the team_id value
            $loadeduser->save(); // Save the user
            
            // Check if user has been updated
            if($loadeduser->field_team->target_id == $team_id){
                return true;
            } else {
                // If it hasn't return false
                return false; 
            }
            
        } else {
            // If it hasn't return false
            return false;
        } 
    }