In the SQL Server 2008 setup you can now set the default backup location but in case you set it wrong or need to change it at later time the SSMS interface does not provide you with any options to do that. You can do the same thing as in 2005 …

Go to following key in Registry and put the new path in …

First we need to determine the instance Name; go to
[HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\Instance Names\SQL\]

Note down the value for the default instance usually MSSQL10.MSSQLSERVER (note they changed the naming convention of instance names from MSSQL.Instance# to MSSQL10.InstanceID which you enter in at install time).

Now go to …
[HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQLServer\]

And change the value for BackupDirectory to the new value.

Now if you modify a Maintenance Plan it will grab the new value. You don’t need to restart server or services.

source: http://sqllearnings.blogspot.com/2009/03/default-backup-path-in-sql-2008.html