I barely got into NPCs and I have problems into creating them.
You aren't supplying much information, so I'll assume that the the arms are CanCollide false and the NPC has no joints.
Use :MakeJoints() on the NPC Model to create joints for it, which should stop the arms falling off.
just create a script inside the arms and inside it put this:
local function anchorNPC --Name this anything you want script.Parent.Anchored = true end
EASIER WAY:
script.Parent.Anchored = true