Microsoft has released Office Web App Server SP1 http://www.microsoft.com/en-us/download/details.aspx?id=42021 which includes all previous rollups released.
Now to install this update, like the previous rollups, you must first remove the Office Web App Farm, then apply the service pack, reboot and re-create the farm.
This is documented at http://technet.microsoft.com/en-us/library/jj966220.aspx
Once you are done this wait about 15 minutes and run a Get-OfficeWebApsMachine in a elevated windows powershell and ensure the server reports a healthy state. It could take up to 30 minutes for this to occur so be patient.
For a great resource on resolving unhealthy Office Web App servers check out Doug Deitterick’s Blog on things to check for.