Stop Icinga and ido2db daemons (since otherwise a new instance would automatically be added to DB instead of renaming it)

 #> /etc/init.d/icinga stop
 #> /etc/init.d/ido2db stop

Change the instance name in /usr/local/icinga/etc/idomod.cfg

 instance_name=newinstance

Change the instance name in the database table "icinga_instances" or "instances"

MySQL/PostgreSQL

 SQL> UPDATE icinga_instances SET instance_name='NEWNAME' WHERE instance_name='OLDNAME';

Oracle

 SQL> UPDATE instances SET instance_name='NEWNAME' WHERE instance_name='OLDNAME';

Change the instance name in the command pipe configuration in one of these files (upgrade safe file in first place)

Clear the Web Cache

 #> /usr/local/icinga-web/bin/clearcache.sh

Start Icinga and ido2db Daemons

 #> /etc/init.d/ido2db start
 #> /etc/init.d/icinga start