Follow these steps below to update theme via FTP:
Step 1. Login your Envato account. In Download tab, find Healthify and choose the Installable WordPress Theme(the WordPress file).
Step 2. Login your server and go to wp-content/themes folder to backup your Healthify theme folder if needed. The contents on your site will not be lost but if you have modified the code then the changes will be lost. You have to reapply them again after updating.
Step 3. Extract healthify.zip that you downloaded from themeforest.net
Step 4. Upload new theme folder healthify to wp-content/theme to override the existing files.