How do I rename columns on a List?

You can rename List columns in the column settings. 

Click the three dots on the top, far-right.

Then select "Columns".

1. Rename List Columns

From here you can rename your List Column headings by clicking on the title and renaming it.

Click "Save & Close".

2. Rename Field Columns

Columns that are tied to Card Fields cannot be renamed here. You would need to update the Field's names in Project Settings, which will change the Field name on Cards and across the Project. 

See How Do I Edit a Field? for more information.