You will have to install Symantec Endpoint Protection Manager, NAV Manager cannot be upgraded to Symantec Endpoint Protection Manager.
There is a folder in the CD/DVD called CompetitiveUninstall. This folder contain utility which can be used to uninstall NAV Clients and Install SEP Clients.
Path: Symantec_Endpoint_Protection_11.0.5_AllWin_EN_CD2\Tools\NoSupport\CompetitiveUninstall
There is a documentation also available in the folder
Symantec_Endpoint_Protection_11.0.5_AllWin_EN_CD2\Tools\NoSupport\CompetitiveUninstall