How to set compatibility level in sql server

from sql server 2005 onwards there is no support for non ANSI outer join operator *= and =*, default compatibility level for
 database created in different versions of sql server are as follows
sql server 6.0   60
sql server 7.0   70
sql server 7.5   75
sql server 2000  80
sql server 2005  90
sql server 2008  100
when you change compatibility level of databse created in 2005 and 2008 to 80 or lower then you can use non ANSI outer join
 operator and for this use procedure sp_dbcmptlevel<database name><level>