Never a bad idea to consult the documentation. Guessing at the source data type and NULLability; you can fill in the table/column names:
ALTER TABLE dbo.TableName ALTER COLUMN ColumnName
NVARCHAR(255) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL;
If you can't check the documentation, Management Studio will sometimes show you the smart way to do something (though it doesn't always choose to do things the best way). Go into the design screen, change the collation, and instead of clicking OK, click the Script button.