Stop your game server and then go to your Game server > Wipe Map/Players page and click Confirm.

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.