This can occur if the server is busy. Keep trying to connect and the update should eventually download.

If you need further assistance downloading the update, you will need to contact Microsoft at 1-800-4MY-XBOX, or visit Xbox Support for more information.

UK Contact Information:
0800 587 1102
+44 20 7365 9792 (from a mobile phone)
0800 587 1103 (from a TTY device)
Xbox Support GB
Xbox Support (other regions)