For my Sprint Replenish, to eliminate any nagging FA19-update messages from Sprint, I downloaded ES File Explorer, gave it root access, and used it to text-edit the build.prop file based on the video instructions at
THIS YouTube link, changing any instances of "EK18" to "FA19". I think there were 3-4 instances that I edited. It was easy, and I haven't been asked to update to FA19. But that may not work for Boost if their most recent update is labeled something other than "FA19", in which case you'd need probably need to change any instances of "EK18" or "FA19" to the name of Boost's latest update.