You can rename a column (change the text that appears in the column header) if it contains a derived field or a formula. You cannot rename columns that contain normal fields, and you cannot rename the left-most Entries column.
To rename a derived field or formula column:
You can rename a column as many times as you like.
If you wish to go back to the original name of the column, open its menu and click Revert to…. (The text of the menu item will indicate the original name.)