From the GRUB Menu, Select "Install or Upgrade Existing System"
You can skip this or select media test, if you want to test installation media before installation
If you wish to install a specific version of CentOS for example CentOS 6.4 use the following URL instead of the one displayed in the screenshot below,
To install the latest available version, use the URL as displayed in the screenshot below,
After this stage -- You should see a Graphical Install Interface load up from where you can select the packages you wish to install and disk partition layout to complete the install.