You can either add the plugin in the WordPress Dashboard (Free plugin only), or manually upload the plugin file (both Free & Premium plugins).
For premium version, after you downloaded the zip file from Envato, ensure the file you upload to your WooCommerce store is epow-custom-product-options-for-woocommerce.zip
Follow this instruction for both Free/Premium download and installation.
Update The Plugin
To update the purchased plugin, within 6 months from purchase, you can receive updates automatically on one domain by authenticating the plugin on that domain. Follow this instruction for specific steps.
Or you can update the plugin manually (no expiry date, you can do it for a lifetime). Simply re-download your purchased plugin file in your Envato account – Download list. More info in this instruction.
For free version of the plugin, you can always update the plugin in the WordPress Plugins list. You can see this instruction – IV. UPDATE FREE PLUGINS for a better view.