For anyone who is looking for a way to change the :uploads assigns to set the max_entries property, we can use Map.put to update the entire assigns socket on maybe_update_upload_config/2 , something like this: defp maybe_update_upload_config(socket, max_entries) do config = Map .get(socket.assigns.uploads, :images) new_uploads = Map .put(socket.assigns.uploads, :images, %{config | max_entries: max_entries}) new_assign = Map .put(socket.assigns, :uploads, new_uploads) Map .put(socket, :assigns, new_assign) end