How do you change the datatype of a column in SQL Server?

ALTER TABLE TableName 
ALTER COLUMN ColumnName NVARCHAR(200) [NULL | NOT NULL]

EDIT As noted NULL/NOT NULL should have been specified, see Rob's answer as well.

Don't forget nullability.

ALTER TABLE <schemaName>.<tableName>
ALTER COLUMN <columnName> nvarchar(200) [NULL|NOT NULL]

Use the Alter table statement.

Alter table TableName Alter Column ColumnName nvarchar(100)

The syntax to modify a column in an existing table in SQL Server (Transact-SQL) is:

ALTER TABLE table_name
    ALTER COLUMN column_name column_type;

For example:

ALTER TABLE employees
    ALTER COLUMN last_name VARCHAR(75) NOT NULL;

This SQL Server ALTER TABLE example will modify the column called last_name to be a data type of VARCHAR(75) and force the column to not allow null values.

see here

For changing data type

alter table table_name 
alter column column_name datatype [NULL|NOT NULL]

For changing Primary key

ALTER TABLE table_name  
ADD CONSTRAINT PK_MyTable PRIMARY KEY (column_name)

As long as you're increasing the size of your varchar you're OK. As per the Alter Table reference:

Reducing the precision or scale of a column may cause data truncation.

ALTER TABLE [dbo].[TableName]
ALTER COLUMN ColumnName VARCHAR(Max) NULL

in 11g:

ALTER TABLE TableName
Modify ColumnName DataType;

EG: ALTER TABLE employees Modify BIRTH_DATE VARCHAR(30);

ALTER TABLE [Performance].[dbo].[CRA_283_Violation]
ALTER COLUMN [Throughput_HS_DC_NodeB_3G_Alarm] bit

Try this:

ALTER TABLE "table_name"
MODIFY "column_name" "New Data Type";