CHANGE YOUR MINECRAFT SERVER VERSION.
This page demonstrates to how to change your minecraft server version easily in a simple way.
Change Your Minecraft Server Version.
If you want to know that how to change the version of your paper server, please follow the steps:
Go to Xerin's Control Panel (https://control.xerin.org/) and select your server.
After that, click on startup and write the server version you want under the "Minecraft Version" section, like I want 1.20.4 so i will type that.
After that go to Settings Tab, scroll down & click on reinstall server and reinstall it.
After reinstalling, start your server and,
You have successfully changed your server version!