The theme can be updated in 2 ways :
1. Semi-Automatically : You should install this plugin : Easy Theme and Plugin Upgrades , download the latest installable WP theme zip file from your downloads page and then upload it via Appearance > Themes > Add New > Upload Theme and upload the zip file. The “easy upgrades” plugin will backup the old theme and install the new one.
3. Manually : You should download the latest installable WP theme from your downloads page, rename your current theme folder in wp-content/themes folder, unzip the downloaded WP theme from your downloads page and upload it to wp-content/themes folder.
Please note that updating a theme will override any custom changes that you make to the parent theme. Hence we recommend using a child theme. A child theme is not meant to be updated and should contain only your custom changes.