1. You can't. When you see people selling demons, they're sitting in a game with an untamed demon ready to be tamed. You can't swap them around like pokemon, sadly.
2. You have the best demon already. Only way to improve is to get specific attributes like enchanted with lightning or fire enchanted.
imo a better demon would be ismail vilehand cursed with HOly shock, holy freeze or holy fire. all because those proc handfuls of enemies at a time. so a whole screen of cursed instantly pops with a holy . its better. it can be on hephisto just holy freeze or cursed with a holy is better imo.
Thank you very much
Have a great day