If your VM is in the 'saved state' first you will have to the discard the saved state to make any changes. To do so in your VirtualBox Manager go to Machine and then select 'Discard saved states' or press 'CTRL+J.' As the prompt indicates this will be equivalent to switching off your system and now you can make the required changes.
Now go to File menu and select 'Virtual Media Manager' or press 'CTRL+D.' Release the snapshot displayed with a red mark and also Delete it.
Now you will have to select the original 'vdi file' that was created during your installation process. To do this Select Modify and go to the location where you have installed your VirtualBox and select the 'vdi file' and Restart.
Your problem should be RESOLVED now.