If you need to update a column in a table than you can use different approaches:
1) To modify a datatype of the column:
alter table Person modify COLUMN_A char(2) null
2) To make a column required:
First look if the column has some constraints:
sp_helpconstraint Person
If the column has constraints than you need to drop them first:
alter table Person drop constraint FK_PERSON_REF_ADRESS
Then if you can specify a default value for the column you can drop the column and create it again:
alter table Person drop COLUMN_A alter table Person add COLUMN_A char(2) default 0 not null
But there is not the way we always need it. In such a case we need to drop the hole table and create it again:
drop table Person create table Person( COLUMN_B numeric(9,0) null, COLUMN_C char(2) not null, COLUMN_D date not null, COLUMN_A numeric(9,0) identity, constraint PK_PERSON primary key (COLUMN_A), constraint FK_PERSON_REF_ADRESS foreign key (COLUMN_B) references Adress (COLUMN_ADRESS_A) )