SQL Server: Update Existing Column Value in Serial Number

I have the below table in my SQL Server Data Base:



Table script:

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

GO

Data in Table:



Now my requirement is below:



So I wrote the below SQL Statement:

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

SELECT*FROM EMPLOYEE_RANK