Hi nabilabolo,
Check this example. Now please take its reference and correct your code.
Database
I have made use of the following table Customers with the schema as follows.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.
Download SQL file
SQL
DECLARE @first INT,@last INT
SET @first = 1
SET @last = 5
DECLARE @Counter INT
SET @Counter = @first
WHILE (@Counter <= @last)
BEGIN
	IF EXISTS(SELECT CustomerId FROM Customers WHERE CustomerId = @Counter)
	   -- Update 
	   UPDATE Customers SET Country = '' WHERE CustomerId = @Counter
	ELSE
	  -- Insert 
	  INSERT INTO Customers VALUES ('test','test')
	SET @Counter = @Counter + 1
	CONTINUE;
END
SELECT * FROM Customers
Output
| CustomerId | Name | Country | 
| 1 | John Hammond |  | 
| 2 | Mudassar Khan |  | 
| 3 | Suzanne Mathews |  | 
| 4 | Robert Schidner |  | 
| 5 | test | test |