Talk to Sophie in the library (Room on the left from the entrance), she will give you the ability to open the silver chests located all over the world. You can then trade the souls you acquire from the silver chests to upgrade the mansion. Every 3 souls you acquire can be used to upgrade another part of the mansion and there are enough souls in the game to upgrade most, if not all, of the rooms in the mansion. 

Alternatively, if you can't be bothered to look for the soul chests in the game, just go to the debug room in Damia's Temple (the first temple at the start of the game) on the rightmost area/sign with the sparkles and talk to the NPC inside that talks about the soul cap. Say yes. Then talk to Sophie multiple times until you upgrade all the parts of the mansion.