Hi,
Thank you for posting in Symantec community.
I would be glad to assist you here.
Symantec Endpoint Protection (SEP) adds support for Windows 10 with 12.1.6 MP1. For Symantec Endpoint Protection 12.1, a maintenance patch has been released on July 29, 2015.
You can directly upgrade from SEP 12.1 RU4 to SEP 12.1 RU6 MP1a.
MP1a version will be available at Fileconnect. Do you still see SEP 12.1 RU5? Are you on SEP.cloud version?