How apply a formular to a column containing data

How can i apply a formular to a column that already contains data once?
I know i can create a trigger formular that is applied to newly created entries.
But i already have some data in a column and i want to clean it up.

Concrete use case:

I have a column containing email addresses, i want to apply the LOWER(value) formular once, to normalize the emails.

I JUST found it, its in the lower right corner:

Apply Formular to Data