How To Update Drupal Themes

Step 1: Download Theme

Download the latest version of theme you wish to update.

Step 2

Put your website in Maintenance mode.

Navigate to:
Home Administration Configuration Development Maintenance mode

example.com/admin/config/development/maintenance

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.

Step 4

Take your site out of maintenance mode.

Step 5:

Clear Drupal cache.

Navigate to:
Home Administration Configuration Development Performance

example.com/admin/config/development/performance

And clear all caches. You do not need to run update.php for theme update.

Done!!!