In order to update the theme via WordPress manually, you can follow the instruction below:
Step 1: Download the Installable theme from ThemeForest in the Download menu on ThemeForest. Then extract it to your computer.
Step 2: Go to Appearance > Themes and deactivate current theme. To deactivate, simply switch to a different theme. For example, the default WordPress Twenty Seventeen theme.
Step 3: If you translated the theme and the language files are in themes/theme_name/languages/. You need to upload them to wp-content/languages/themes/. If not, after updating the theme, any customizations to the theme’s core files, such as PHP files, language files will be lost.
Step 4: After deactivating you can go ahead and delete it. To do this, hover over the theme thumbnail then click Theme Details. In the bottom right corner of the window, click the Delete button. All your content such as pages, options, images and posts will not be lost or erased by doing this. However, any customizations to the theme’s core files, such as PHP files will be lost. So you need to the child theme to customize the theme.
Step 5: Upload, Install and Active the theme.
Step 6: Don’t forget to update the required plugins