If you want to reset a specific player's progress or start over players in an existing world, you can use these steps to remove the player data from your server.
1. Open the File Manager
2. Navigate to /minecraft/<worldname>/playerdata/
Note: If you have not changed your world name, the worldname is Indifferent_Broccoli_Minecraft_Server
3. Delete the .dat
files for each UUID of the players you want to reset.
4. Restart the server.