How to modify or insert already existing table value using by another table in sql server?

In this article, I describe How to modify or insert into already existing table value using by another table.This is achieved by a select statement. As here I am select top 10 rows from dbo.Employee table and insert into dbo.Employee_Test Table.

INSERTINTO DBO.EMPLOYEE_TEST

SELECT TOP 10 * FROM DBO.EMPLOYEE 

Here, DBO.EMPLOYEE_TEST is already existing table in the database; we inserted top 10 rows value from dbo.Employee table