“Outdated server” or “Outdated client” means your game version and the server version are different.

Option 1: Match your game to the server

1. Open the Minecraft Launcher and go to Installations.
2. Create a New installation and pick the exact version your server runs.
3. Save and launch that installation.

Option 2: Change the server version

Change the server to the version you want in the panel, or reinstall it if you are changing server software. Make a backup first. See Updating Your Server.

Option 3: Let more versions join

Install ViaVersion and ViaBackwards so players on other versions can connect. See Let Players Join From Any Version.

Also check

• Bedrock players need Geyser to join a Java server, see GeyserMC.
• Some modded clients show this when mods do not match the server.