Follow these steps below to update theme via FTP:
Step 1. Log into your Envato account on themeforest. In your Download page, find MegaBuilder theme and choose the Installable WordPress Theme.
Step 2. Login your FTP account via FTP software to access your host web server. Navigate to wp-content/themes folder to backup your current megabuilder 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 modify them again after updating.
Step 3. Extract megabuilder.zip that you downloaded from themeforest
Step 4. Upload new theme folder megabuilder to wp-content/theme to override the existing files.