Before you start updating your site always do a full backup of your website.
Erlega is build as a plugin so it's very easy to update. Simply download the latest version from your gumroad library, unzip it, navigate to the theme-files directory and upload it's content to your web root. When asked to replace/override exisisting files choose yes.
Usually only the plugin folder erlega-theme needs to be updated but in some cases the below folders will also need to be replaced/updated.
assetsfoldersite/pluginsfolder