Kind of depends on if you mean Custom NPCs or the Vanilla NPCs?
For Customs NPCs someone else has made, you can change their starting relationship with the player and what their relationship is with other NPCs, both custom or vanilla, and edit their knowledge by adding or removing what's already there.
For Vanilla NPCs you can only add backstory knowledge to what they already have. Everything else, their starting relationship with the player and their relationship with other vanilla NPCs isn't accessible. At least not without using mods, which you can check on the unofficial discord server. I think we have a mod that let you change vanilla NPCs starting relationship with the player, and a mod or two that might let you see and edit vanilla NPC knowledge, routine activity strings, and the hidden short descriptions on walls or floor tiles you can't normally see, but the LLM can. I'll see if there is one for changing the static relationships between Vanilla NPCs.

