How to update a Minecraft server safely

New Minecraft versions bring new blocks, creatures and fixes, and players on the newest launcher will want to join. Updating a server is not hard, but one thing makes it risky: a world that has been opened in a newer version cannot be reliably opened in an older one again. That is why the first step is always a backup.

Before you update

Make a backup of everything, the world folders, the plugins folder and the settings. See how to back up a world. Keep it somewhere other than the server.
Check your plugins. Look up each plugin's page and see whether it supports the new version. A plugin that has not been updated yet is the most common reason an update fails. Wait, or replace the plugin.
Check Java. New versions sometimes need a newer Java. The Java version checker tells you which.
Pick the right moment. Update when few players are online, and tell them in advance.

Updating step by step

Stop the server.
Download the new server jar (for Paper, the newest build of the new version) and put it where the old one was. Keep the old jar until you know the update worked.
Update the plugins that have new versions, and remove any that are not compatible.
Start the server and read the log. The first start upgrades the world, which can take a few minutes for a large one.
Join, look around, and test the plugins you care about.

If it goes wrong

The server will not start. Read the log. A red plugin, a missing Java version or an out-of-memory error are the usual causes.
A plugin misbehaves. Remove it, restart, and wait for an update.
The world looks wrong. Stop the server and restore your backup. Do not keep playing on a damaged world.
You want to go back. Restore the backup you made before the update. Starting the old version on the upgraded world is not safe.

Going from an old version to a much newer one

Jumping many versions at once (for example 1.12 to 1.21) can break builds, because the game changed how it stores blocks and worlds several times. It usually works world-wise, but plugins and datapacks often do not. Test on a copy first, and do it in a few steps if you can.

On Saryno

Open the server, go to Settings, and switch version. Saryno keeps your world, plugins and settings, installs the matching Java, re-resolves your add-ons for the new version, and keeps a copy of the world from before the switch so you can roll back. Take a backup first anyway.