Many virtualization platforms nowadays support the conversion of virtual machines saved in OVA format (Open Virtual Machine Format) to OVF format (Open Virtualization Format Package), which is what ova to ovf conversion represents.
Easiest way is to download some free program like VirtualBox which is fully capable of ova to ovf conversion.
- Select File menu ► Import Appliance... ► click through the import process and import .ova file
- Select File menu ► Export Appliance.... select the imported virtual machine and export it through wizard as .ovf file.