Changing the version variable on the Startup tab doesn't change your version. It's only read when the server is first installed, and a reinstall leaves the old version's files behind. The Minecraft tab does the whole job properly.
- Open your server in the panel and go to Minecraft → Versions.
- Pick the version you want. The list only shows versions your server type supports.
- Choose whether to set your
modsorpluginsfolder aside. Do this when going between major versions, because mods built for one rarely load on another. - Start the switch. The server is stopped (and backed up, if it has backup space), then the new version is downloaded and checked before anything is replaced.
- When it finishes, start the server.
Going back to an older version
Opening a world on an older version than it was last saved with can quietly corrupt it. If your world was last opened by a newer version, the tab stops and asks. You can start a new world (the old one stays on disk) or accept the risk yourself.
What you can't change
The loader (Paper, Fabric, NeoForge and so on) is fixed by the type of server you created. Loaders need very different amounts of memory and CPU, and each has its own allocation. To move to a different loader, download your world, delete the server and create one of the type you want.