How to Set your Minecraft Server to Offline Mode
BEFORE DISABLING ONLINE MODE:
Disabling online mode means that your server will no longer use Mojang authentication for Minecraft accounts.
This means that players using cracked Minecraft clients can join your server. By using these clients, players can get whatever username they want. This can result in any players using your username for gaining permissions to the server.
Also, you may experience a player data reset when switching from online mode to offline mode, as all players get a new UUID.
These are the steps to follow:
1. Go to your server's control panel.
2. Click on the "Console" section.
3. Scroll down to the bottom of the page, you will find the "Disable online mode" button, click on it.
4. Restart the server.