SQL Server - Update existing column value in serial number

In this article I am going to show how we can update our existing column value with serial number without using loop. Below is my problem.

I have below table in my SQL Server Data Base:

Image 1.

Table script:

CREATE TABLE [dbo].[Employee_Rank](
          [EmployeeName] [varchar](50) NULL,
          [Rank] [int] NULL
) ON [PRIMARY]
GO
 

Data in Table:

Image 2.

Now my requirement is below:

Image 3.

So I wrote below SQL Statement:

Image 4.

DECLARE @SRNO INT =0;
UPDATE EMPLOYEE_RANK SET @SRNO = RANK = @SRNO + 1
SELECT * FROM EMPLOYEE_RANK