create a trigger in sql on update for taking previous row backup in another table.(like maintain the history of the record)

Suppose you have two tables one is Employee and another is Employee_Backup.

Structure of Employee table:

CREATE TABLE Employee
(
 Emp_ID int identity,
 Emp_Name varchar(55)
);

Structure of Employee_Backup table:

CREATE TABLE Employee_Backup
(
 Emp_ID int,
 Emp_Name varchar(55)
);

Now create a trigger on table Employee table:

CREATE TRIGGER trgAfterUpdate ON dbo.Employee
FOR UPDATE
AS
DECLARE @empid int, @empname varchar(55);
SELECT @empid = i.Emp_ID FROM inserted i; 
SELECT @empname = i.Emp_Name FROM inserted i;

INSERT INTO Employee_Backup(Emp_ID,Emp_Name)
VALUES(@empid, @empname);

Execute query like below:

UPDATE Employee SET Emp_Name = 'YourName' WHERE Emp_ID = 1;

Now you can see one record will be inserted in Employee_Backup Table.