Step 1. Log into your Envato account on themeforest. In your Download page, find Teching 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 teching 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 teching.zip that you downloaded from themeforest
Step 4. Upload new theme folder teching to wp-content/theme to override the existing files.