How To Update Themes In Drupal 8
Step 1: Download Theme
Download the latest version of theme you wish to update.
Put your website in Maintenance mode.
Home Administration Configuration Development Maintenance mode
And put you Drupal 8 website in Maintenance mode
Step 3: Upload Theme
Upload new theme files by overwriting the existing files.
Custom themes are located in /themes directory.
Take your site out of maintenance mode.
Clear Drupal cache.
Home Administration Configuration Development Performance
And clear all caches. You do not need to run update.php for theme update.