So just to be clear.
The section "Downloading Minecraft Server" gives a wget command specifically for downloading V1.14.1. You need to check the minecraft download page for the url of the latest version (https://launcher.mojang.com/v1/objects/ ... server.jar
at the moment) and use that in your wget command instead like this:
wget https://launcher.mojang.com/v1/objects/bb2b6b1aefcd70dfd1892149ac3a215f6c636b07/server.jar -P ~/server
I think you need to have su'ed to the minecraft user to make sure the file has the right permissions before downloading, I haven't actually read all those instructions.
After you've downloaded the new version I think all you need to do is return to your admin user restart your systemd job
sudo systemctl restart minecraft
There might be some kind of minecraft specific stuff you need to do like changes to the config files but probably not, you can check the release notes to be sure if you really want to.
I wasn't trying to be snarky when I asked for the instructions you followed but without them it's pretty hard to help, there are probably a hundred was to install minecraft on linux.