This is not my case.
As I mentioned, I don't have altbootbank with update 2. This is because after update 3, I also installed the latest patch.
So both bootbanks now have update 3.
Is there any other option for me. Can I just install update 2 again so it will overwrite everything? Or it is impossible?
Backup ESXi configurations and reinstall it without overwriting VMFS.
Downgrading ESXi to prior version is not possible. Only thing is to restore to previous version is to use Shift + R in DCUI , in your case without altbootbank that wont work.
Take ESXi configuration back up using the steps in the article VMware Knowledge Base
Reinstall the OS and restore the config backup.
Thank you very much for the replies