To upgrade the Office Web Apps 2013 farm server to December 2014 CU (Build number 15.0.4673.1000) use these steps.
After upgrade installed successfully, then run thse PowerShell script lines on the OWA farm server:
Script line (replace with your URL and certificate name):
New-OfficeWebAppsFarm -InternalURL “<your OWA farm url, preferrably https>” -CertificateName “<your certificate name>”
Script line (replace with your URL):
Set-OfficeWebAppsFarm -ExternalURL “<your OWA farm url, preferrably https>”
Note: There is no need to re-create the WOPI bindings in the SharePoint 2013 farm.