Redownload the systemupdate17559.usb.zip file, as the initial download may have been corrupted.
systemupdate17559usb.zip is the official offline installer for the Xbox 360 Dashboard Update 2.0.17559.0 systemupdate17559usbzip
Here is a full feature breakdown of what this update entails, why it exists, and how it is used. Redownload the systemupdate17559