VMS Help NCS Command, Command Qualifiers, /REPLACE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  /REPLACE

   This is the default library  operation  if  you  do  not  explicitly
   specify a qualifier. Use this qualifier to have NCS replace one  or
   more library modules with definitions from the source  input  file.
   If a replacement module in the source input file does  not  have  a
   corresponding module in the library, the LIBRARY  command  inserts,
   rather than replaces, the new module in the library.