All our theme updates are available in your ThemeForest.com profile for free: simply redownload it in “Downloads” tab.
At the first step we advise you to rename your current theme folder to something like: …/wp-content/themes/dt-THEME_NAME-old/. It will help you to save custom changes of the template.
At the next stage you need to install the theme in the usual way.
All theme settings are stored in a database, in an array, which is named in accordance with the theme name. Theme settings are not affected by theme files. So you won’t lost any custom settings.