CREATE TABLE [dbo].[sample](
[Unique_ID] [bigint] NULL,
[Person_ID] [nvarchar(max)] NULL,
[Product] [nvarchar](max) NULL,
[Version] [bigint] NULL,
[Count] [bigint] NULL
)
To alter the column Datatype
ALTER TABLE sample
alter column person_id bigint
To Set Default Value for the existing Column
alter table sample
add default 0 for person_id
:)