Step 1. Login you Envato account. In Download tab, find sailing and choose the Installable WordPress Theme (the WordPress file).
Step 2. Login your server, then go to wp-content/ themes folder and backup your sailing theme folder if you want or delete it. The content in your site will not be lost but if you have modified code, the changes will be lost. You have to reapply them after updating over.
Step 3. Exact new file sailing.zip you have already downloaded from Themeforest.
Step 4. Drag & drop new theme folder sailing to wp-content/ theme to override the existing files.