How to replace a column of values in Excel

FIRST ANSWER
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.

SECOND ANSWER
If they are consecutive, you can always:

1. click the first cell of the column.
2. hold ctrl + shift
3. press the down cursor/arrow in your keyboard

This would select all the column at once.

Then apply my first answer...

Job done.

Good luck!