There are two butterflies in The Witch's House. One is a blue butterfly model and one is a yellow butterfly.
Yellow butterfly
It is located in the Barrel room on the 2nd floor.
You have to save the Yellow Butterfly from the spider web to be able to continue to floor 3. If you just take the Yellow Butterfly from the web then the spider will kill you. To avoid that you need the Blue Butterfly Model and replace it with the Yellow Butterfly.
Blue butterfly model
It is located in the Glass Display room on the 2nd floor.
You have to replace it with the Yellow Butterfly inorder to save the Yellow Butterfly. But it's not easy to get the Blue Butterfly Model since you have to get it out of the glass case, how you do that is to put the Book of Death on the Bookshelf. Then it will unlock.