I'm sure there will be a formula for that, but as I'm not aware of that formula or macro, you could do it in the easy way:
Copy all the column to another excel.
Simply replace all the N to S.
Copy the column back to the original excel.
If they are consecutive, you can always:
- click the first cell of the column.
- hold ctrl + shift
- press the down cursor/arrow in your keyboard
This would select all the column at once.
Then apply my first answer...