As you might know, when Microsoft originally released Service Pack 1 (SP1) for Office Web Apps Server 2013 in April 2014, we discovered a flaw which prevents any public or cumulative updates from being installed.
Because of this flaw, we immediately removed the ability to download the service pack so we could repair it. More on that here... http://support.microsoft.com/kb/2817431
Once the regression was patched, we published a new KB with the patched version of SP1. That KB can be found here...http://support.microsoft.com/kb/2880558
If your Office Web Apps server is running on the regressed version of SP1, it is highly recommended you remove it and install the patched version.
The patched version of SP1 is build 15.0.4571.1502.
Read more at source: http://blogs.technet.com/b/office_web_apps_server_2013_support_blog/archive/2014/06/05/how-to-ensure-you-39-re-using-the-patched-version-of-sp1-for-office-web-apps-server-2013.aspx