how do i get serana to become human

you can't, she said that she likes the gift that she has and will not get rid of it.

Actually you can, by just not broaching the subject before finishing the MQ, and choosing the aid the Dawnguard.

However...

...all you are likely doing is speeding her to Coldharbour and Molag Bal's... erm... 'hands'. I imagine that Vampires go to his realm like werewolves go to Hircine. However, even cured she was a Molag worshipper and went through a 'special' ritual to becoem a pureblood. I have a feeling that even after being cured the initial contract still stands. He owns her soul.

I would prefer to keep her as a Vampire, at worst to keep her from him as long as possible, and best to find some stronegr and better place to have her soul destined to go.

oh well i only played the vampire one soooo.... yeahh