The easiest way to update Drupal 8 core on your website is to use composer. Before you give it a try, make sure you have composer installed on your server and your Drupal webbsite.
You can find a step by step instrcutions here: Update Drupal 8 core
Migrating from lagecy/older version of Drupal to Drupal 8 is a bit tricky and cumbersome to perform when it comes to importing data and retaining all the relations among them. We have published this guide to help you migrate your Drupal data to Drupal 8 using Drush.
I have built the Drupal 8 website with all the features and theme. Now, it is time to migrate the data, users and everything else to this newly built Drupal 8 website. How can achieve this migration of data from lagecy/older version of Drupal to Drupal 8 website?