To download the Theme latest version: http://www.themeforest.com>Your Account>Downloads. There you can find our latest Theme version.
To install it: Use the same procedure as when you installed the theme for the first time.
Everything you did on your Dashboard won’t get overwriten, the files you’ve modified, however, will be replaced with the new files.
If you have made any code changes in the files previously then I recommend that you save them in separate folders so you’ll have them at hand to reapply the changes.