it is possible to rename the named instance of SQL server 2K, first use sp_dropserver and sp_addserver (drop the old server name and add new server name in sysservers table)....

Thanks Amit,

but I'am unable to understand, why my query is not working..what might be wrong with it

After sp_dropserver sp_addserver  you have to restart the sql server for that global variable @@ServerName to take effect.