DB2 doesn't start after change db2 hostname
You could just try applying those steps. 
If that fails, another option would be to create a new instance and then restore the database(s) over to the new instance. 
Another option could be uninstalling the instance and then reinstalling it. 
Another could be taking a backup of the database, saving it off somewhere and then doing an image restore (pending you have one of those) on the system.


If it were me, I'd just try applying the steps in the rename the DB2 hostname link first. Then I'd probably go for either creating a new instance or uninstalling and reinstalling the existing instance.