Which episode does nell transforms - bleach?

It seems you're referring to the transformation of Nel Tu, the Arrancar in the anime Bleach. However, Nel doesn't actually *transform* in the traditional sense. Instead, she recalls her past memories and regains her full power.

This happens in the Bleach anime, episode 294, titled "The End of the Hollow". In this episode, Nel is gravely injured and seemingly on the verge of death. However, she remembers her past life as Nelliel, the former 3rd Espada, and her full power awakens, allowing her to heal and defeat her opponent.

It's important to note that Nel's transformation is more about regaining lost memories and power rather than a physical change like some other Arrancar.