Hi All,
Now I successfully installed Oracle apps. How do I backup the existing Oracle database so that I can refresh the instance anytime? Thank you and your help is much appreciated

There are three options. I would go with option 1 or 2.

Option 1: Shut down VM machine. Copy the folder in which VM is installed into a safe place (external drive). When the rainy day comes, you can reinstall the VM software and re-register the back up folder on as a new virtual machine.

Option 2: Take the snapshot of your virtual machine from VM control panel. You can roll back the snapshot when something goes wrong with linux system. However the virtual machine software should be still available on the host system though. You can do combination of option 1 and option 2. Do option 1 once and option 2 periodically. Or you can do option 1 alone periodically. Option 1 will protect you in case of hard disk failure as your back up is residing in external drive. Do perform a test recovery run though.

Option 3: Take database level back. Copy data files and control files into a separate folder in linux or to external drive. You can roll back to the previous state by restoring the backup. You can use combination of 1 and 3 as well. If there is a total failure then you can use option 1 to restore your VM back to the point of fresh installation and then restore database back by using option 3 to a point of your recent OEB configuration. You need to have technical background for option 3.