Tenshi na Konamaiki

Megumi-chan is a girl with a secret past.

She used to be a boy until she met a person she thought was a magic user.

This person gave him/her a magical book from which a genie appears to grant one wish when blood is applied to it.

Megumi made the wish to be a man in a man's body but the genie has a twist: he grants wishes backwards so he turns Megumi-kun aged 9 to Megumi-chan.

Years pass and Megumi enters High School where she immediately beats up the school bully who of course falls in love with her.

She is looking for that book again to be able to reverse the spell placed upon her.