Find/Replace VBA for a specific column

I want to run several find and replace commands. I'm not sure how to limit the scope of the command to a specific column.

I tried this macro to replace all spaces in column A with the letter N but the command runs without making any changes.

Sub reppp()
Columns("A").Replace What:=" ", Replacement:="N", SearchOrder:=xlByColumns
End Sub

Thanks so much!