Modify formula in custom column Power Query

Hello,

I appreciate help as I'm new in Power Query world of learning.

I created a custom column in Power Query with a simple formula. If I'd like to go back and edit this formula, how would I do that. I see the formula in a formula bar but it doesn't look in a structural format (say Column A + Column B = Column C).

I tried to look up all commands in edit mode but couldn't find how can I get to the window where I initially was setting up the formula in that custom column.

thank you everyone in advance!

In the picture below an example: if you click on the gear button at the right of the step name (the one in the red circle), then you get the pop up in the middle.



Power Query is pretty structured, but maybe in another way then you are used to.