Once you are there, we recommend you to start your server for the first time if you didn't do it for now
If it's your first time turning on your server you need to accept the EULA
Once you have done this your server start for the first time, let it a few time to generate the world. (In diamond it takes 29 seconds for example) It's only once so you will only wait once. It depends on your plan. Once your server is marked as "ON", you are ready to play or in this case change version. (Between you are not forced to do this step, but it's recommended to prevent issues. If the map is not totally generated for example). From default, you have the vanilla server.
Now go under configuration -> Advanced
Choose the edition and the version you want for example spigot 1.15.2 and press install edition.
Wait 10-20 seconds and go back to start up your server! Have fun!