It is very likely that you’re already running a supported version. But if you want to verify it you must open a terminal on the machine on which your server is running on. If it’s a Windows machine click on the Windows symbol and search for bat, cmd or powershell (Win10+ only). Otherwise, if it’s a Linux machine open your terminal. It is possible that you need a software, such as PuTTY, WinSCP or Bitvise, to connect to your distant machine.
If you’re inside the console type java --version to display your currently used version. If it’s printing that the command is missing then your machine isn’t even viable to run the Minecraft server. Otherwise, it should print something like this:

It should print either Java 8, Otherwise, you’ll have to update it to at least version java version.