In order to switch your server version you will have to do these things in the following order:
1) Download your target server version
In order to select your server version you need to have it downloaded to your computer. We suggest downloading the ".jar" file from https://getbukkit.org
2) What type of jar to select
Ideally for a regular Minecraft server that you are planning to run plugins on you want to download a jar from the "Spigot" dropdown. For players that know what "Paper" is you are also able to find said ".jar" files on the website linked above!
3) Upload the jar
Once you have the server ".jar" downloaded to your computer you are going to want to open the "File Management" section of your server's control panel (https://mcp.pixelhost.org), and you are going to want to simply upload the jar to the main directory of the server where you should also see a file currently named "server.jar". If you don't see a file named "server.jar" you have either already renamed it, deleted it, or have already set up your server correctly.
If you do happen to see "server.jar" on your server's main directory already, perfect, you are halfway there! Now all you have to do is drag your ".jar" file from your computer into the window where the files are or simply hit upload and select your file!
4) Delete the old "server.jar"
Now that you have uploaded your new jar, you can proceed to delete the default server version "server.jar"
5) Rename new server version
Lastly, once you have completed all the steps above, all you have to do is rename the new file that you uploaded to "server.jar" and you are ready to play!
Warning: If you plan on switching your server from a newer server version to an older server version, ensure that your world files are not using incompatible blocks or the server will crash on startup! i.e switching from 1.15.2 to 1.8.8 with 1.15.2 blocks in your "world" file, your server will not be able to start correctly!
If you have completed all the steps above you are ready to go with your brand new server version.
The PixelHost Team