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.
You can find the build number of your Office Web Apps server in Control Panel > Programs and Features
If you are on an older build than this, it means you are using the regressed version of SP1, or you are on an older Cumulative Update (CU), or an older Public Update (PU), or on the default release (RTM) build. In any situation, it is highly recommended that you upgrade to at least SP1.
There is also the situation of the PUs & CUs which have been released since the patched version of SP1 was released. Here are the major points to keep in mind. Download links at the bottom of this section.
-If you have the regressed version of SP1 for Office Web Apps server installed, please reference Jason Haak's blog on how to how to remove the regressed version of SP1, install the patched version of SP1, and install April PU. http://blogs.technet.com/b/office_web_apps_server_2013_support_blog/archive/2014/04/08/how-to-remove-office-web-apps-2013-with-service-pack-1-and-apply-security-update-2878219.aspx
-Since the June CU is right around the corner, the upgrade path MUST be done in the following order to be completely current with updates. Patched SP1 > April PU > May PU > June CU.
-Generally when updating Office Web Apps Server, you can install the latest PU and CU, as each PU and CU are roll-ups of all other PUs and CUs before it. However, in this unique situation, because of the regression with SP1, April PU must be installed before the latest available PU.
SP1 Regressed - http://support.microsoft.com/kb/2817431
SP1 Patched - 15.0.4571.1502 - http://support.microsoft.com/kb/2880558
April PU - 15.0.4605.1001 - http://www.microsoft.com/en-us/download/details.aspx?id=42485
May PU - 15.0.4615.1001 - http://www.microsoft.com/en-us/download/details.aspx?id=42799
June CU - http://support.microsoft.com/kb/2881051
Latest available SP, CU, and PU for all Office/Office related products - http://technet.microsoft.com/en-us/office/ee748587.aspx
As always, please do not use automatic updates to install updates to Office Web Apps Server 2013.
Please follow the best practices outlined in the following TechNet article. http://technet.microsoft.com/en-us/library/jj966220(v=office.15).aspx
Edited 6/11: Updated April PU link. Updated June CU link.