Leave only first character in the column cell

Hi guys, first post here

I’m doing a course regarding data-analytics with Jupyter and I’d like to modify a column cells so, that there would be only the first character left.

For example, if I have a ‘Cabin’ column which contains cabin data like A12, B56, C456. How would I modify the column so, that the cells would show only the letter.

I googled for a while, but did not get definitive answer.