Next you have to reconfigure game files this happens automatically but may take up to a hour to complete. If your downloader seems stuck at a certain percentage don't freak out it's still working. As this step takes so long it's best to get it done this morning if possible.
Then you can update your addons this is a major patch all of your addons will be broken Curse Client is a fast, no hassle way to get this done.
Expect a delay in the estimated time servers come back up, this is a systems patch more than anything. If you expect a delay you'll be less annoyed.
As with every patch we will be here with updates as delays happen and/or servers come up and there will be a live stream of all the new toys.
(If it still doesn't download check out these possible fixes.) http://us.battle.net/wow/en/forum/topic/6412103544
"failed to read a required file."
Delete your Battle.net folders
Try deleting the Battle.net folders.
**Note** you will need to enable hidden files and folders on your system in order to see the Program Data folder.
How to show hidden files!
Ok this didn't work the first time i had him try it, but i had him try again and it worked!
So if it doesn't work the first time for this error try again, it seems to be the required fix.