Create the object of your entity class, add it to the data context using add object method and then call save changes method.

CustomermytestEntities obj = new CustomermytestEntities(); 
Customer objCust = new Customer(); 
objCust.CustomerCode = "1001"; 
obj.Customers.AddObject(objCust); 
obj.SaveChanges();

If you want to update , select the object , make change to the object and call accept changes.

CustomermytestEntities objContext = new CustomermytestEntities(); 
Customer objCustomer = (Customer)objContext.Customers.FirstOrDefault(); 
objCustomer.CountryCode = "NEP"; 
objContext.AcceptAllChanges();

If you want to delete call the delete object method as shown in the below code snippet.

CustomermytestEntities objContext = new CustomermytestEntities(); 
Customer objCustomer = (Customer)objContext.Customers.FirstOrDefault();
objContext.DeleteObject(objCustomer);