Stop your game server and then go to your Game server > Wipe Map/Players page and click Confirm.
Change World Type
Go to your Game server > Settings > server.properties page and change "level-type". It determines the type of map that is generated. default - Standard world with hills, valleys, water, etc. flat - A flat world with no features, can be modified with generator-settings. largebiomes - Same as default but all biomes are larger. amplified - Same as default but world-generation height limit is increased. buffet - Same as default unless generator-settings is set to a preset.