Because GLupeN64 is no longer the "standard" N64 core on the official Libretro buildbot, you typically have to find it through community-maintained archives or specific hardware mod repositories.
If you have downloaded the core file manually (e.g., glupen64_libretro.so or km_glupen64_libretro.so ), follow these steps to add it to RetroArch: glupen64 core download
The most popular version is maintained by KMFDManic . You can find his core sets on his KMFDManic GitHub or via his community-shared Google Drive archives. Because GLupeN64 is no longer the "standard" N64