To set the database compatability level of a database using a t-sql commmand you will use the ALTER DATABASE command.

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ALTER DATABASE [database_name] SET COMPATIBILITY_LEVEL = 90 | 100 | 110
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SQL Server 2000 = compatability level 80
SQL Server 2005 = compatability level 90
SQL Server 2008 = compatability level 100
SQL Server 2012 = compatability level 110

Here is a quick and dirty script to set the compatability level of all the databases on the server:

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declare @dbname varchar(128), @sql varchar(max), @level varchar(3);
set @level = '110' --- 90 | 100 | 110
declare cursor1 cursor for 
select rtrim(name) from sys.databases 
   where databasepropertyex(name, 'Status') = 'ONLINE' 
   and name not in ('master', 'model', 'tempdb', 'msdb', 'distribution');

open cursor1;
fetch next from cursor1 into @dbname;

while @@fetch_status = 0
begin
   set @sql = 'ALTER DATABASE ' + quotename(@dbname) + ' SET COMPATIBILITY_LEVEL = ' + @level + ';';
   exec(@sql);

   fetch next from cursor1 into @dbname;
end
close cursor1;
deallocate cursor1;
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