How to change the Compatibility Level of a Database

 
Transact-SQL:
ALTER DATABASE DBname SET COMPATIBILITY_LEVEL = { 80 | 90 | 100 }

For example,
ALTER DATABASE DBname SET COMPATIBILITY_LEVEL = 90
sp_dbcmptlevel Stored Procedure:
EXEC sp_dbcmptlevel DBname, compatibility_level value;
For example,
EXEC sp_dbcmptlevel DBname, 90;