If you're looking for an easy spell to summon a demon then it is more than likely that you don't yet have the knowledge or the skill to actually do what you want.
And remember, if you summon something up, you also need to be able to put it down when you are through. There is an old adage in High Magick that says "Do not summon that which you cannot put down."
And as Nichiren said, you cannot change yourself into a demon nor a part demon. Nor, for that matter, can a demon turn you into anything other than human.