Re-indenting your code so it is clearer:
create Proc insert_Returns
@RID int output,
if not exists(select ReturnsType from tbl_ReturnSetings where RID=@RID)
SET CSPrefix = @CSPrefix
Makes it a bit more obvious.
But what you are asking for is pointless - your SP does an INSERT if no record exists, or
does an UPDATE if it is already present.
But your insert looks to use an IDENTITY field - in which case you cannot provide an ID value (So are presumably passing a negative value to ensure it does an insert). And if it does an insert, then there is no point in doing the update, because it updates the field you just inserted with the same value are are updating...
I think you need to sit down and think about what you are trying to do, because I'm pretty sure what you have asked for is not it...