Answers for "modify datat type in sql server"

sql server alter column

ALTER TABLE table_name 
ALTER COLUMN column_name new_data_type(size);

sql alter table

-- With check if field already exists

IF NOT EXISTS(SELECT 1 FROM sys.columns 
          WHERE Name = N'Email'
          AND Object_ID = Object_ID(N'dbo.Customers'))
BEGIN    
	ALTER TABLE Customers
    ADD Email varchar(255);
END