If it is a Linux kernel update, then it's either Livepatch or you reboot.

If it is a system library, then for the apps to use the newly installed library, they need to be restarted (sometimes logging out and logging in again should be enough).

If that library is the system "libc" library or something similar, then it has to be reboot (not relogin). The only way that I can think for reboot-free updates, is to save the full state of the system, then silently restart everything to the previous state.