Before you go ahead with the update please check template changelog on the product page. It is the best practice to backup site regularly and especially before making some serious upgrades. We recommend you to use trusted backup component for Joomla! : Akeeba Backup (Core or Pro).
Typical template update process is easy:

From Extensions: Update but request from you filling two fields License Key and email in template "Update Settings" tab.

Anytime you can manually reinstall new template using a new package. Download template from download section and install, it will override all template files. Do not worry custom.css file would be untouched.