How to update a theme ?
First you need to use an FTP client ( like FileZila ) .
You can upgrade Startit theme by performing following steps:
1. Download latest All files & documentation file from ThemeForest (your Downloads page)
2. Extract it and locate startit.zip
3. Extract startit.zip and locate startit folder
4. Copy/Replace the content of theme folder to /wp-content/themes/startit folder of your web site.
Using this method, none of existing content or settings will be lost.