Sacrifice a foal, Use a bit of your blood (Has to be Your old human blood, But it can also any dead skin or whatever), Repeat the following spelling:
“Vér biðjum þik, Óðinn, snú mik aptr í mann; ek varð at hesti fyrir óhapp ok vil eigi enda sem Loki þá einu sinni.”
Eat any sort of magical mushroom, after some three hours, you end up normal.