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 Akeeba Backup (Core or Pro).
The typical template update process is very easy:

• Download new version of a template
• Re-install with it
  OR
• Use the Joomla! update system. But first you have to fill two fields, email and license key here:

Don't worry, during template update - site content, template settings, and custom.css file (used to keep custom CSS code) would be untouched. But we always suggest making a full backup anyway.

After template update, check if used Helix Ultimate Framework is updated as well.