You can use VMWare's OVFTool to convert from OVF to OVA and vice-versa.

1. Download and install OVFTool. Versions for Windows, Mac and linux are available. You'll need a (free) VMWare Account.
2. Collect your OVF (and dependent files) and place them on the machine you've installed OVFTool on.
3. Open a command prompt/Terminal and navigate to the folder where the files are located.
4. Perform the conversion (adjusting command and paths according):
   For OVF to OVA (note that all files referenced in the OVF must be in the same folder):

   install-path\ovftool.exe C:\path\source.ovf C:\path\dest.ova

   For OVA to OVF:

   install-path\ovftool.exe C:\path\source.ova C:\path\dest.ovf