Hi Vizzy
First tell me that your instance that you want to rename contains any
databases in it ? If so take complete backups of the all the database
including system databases.
Then you have to drop that instance and you have to add server again
using sp_dropserver and sp_addserver stored procedures. For
commands' details refer Books OnLine. After issuing these commands you
have to restart the SQL Server Services to complete the process.
What you have to do before that is take care of the linked server that
are already configured and the logins already available there in that
instances. To keep it alive take the system databases' backup.
Refer this link for a script that is available.
http://vstoolsforum.com/blogs/sqlserver/archive/2007/04/10/renaming-a-sql-server.aspx
Regards
Esakkiappan Barathan