how to remove foreign key constraint in sql

USE AdventureWorks2012;  
GO  
ALTER TABLE dbo.DocExe   
DROP CONSTRAINT FK_Column_B;   
GO  

ALTER TABLE table_name
DROP CONSTRAINT fk_name;

ALTER TABLE Orders

DROP CONSTRAINT FK_PersonOrder; 

ALTER TABLE Orders

ADD CONSTRAINT FK_PersonOrder

FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID);