Here you can find some helpful information about Prestashop installation and first steps after that.
Video for the Alysum theme, but process is the same for the Arundo.
You can find Arundo updates in your themeforest account. You can follow us on twitter. We always tweet about updates and releases https://twitter.com/Mnishek
1) Backup your site before updating (it's very important, if the update is done with errors you can always go back to the previous version). Just backup the whole site folder of your Prestashop platform.
2) Disable all the cache. Advanced Parameters > Performance >
Template cache: Force compilation Cache: Disabled CCC (Combine, Compress and Cache):
3) Copy the content of the archive /theme/arundo-v.xx.zip to the site folder with the Prestashop platform.
Here is video tutorial “How manually update the Arundo theme” http://screencast.com/t/8fqEFkT1V6Li