How do i delete a created world

Worlds can only be deleted by manually removing their savefiles at the moment, one of the future updates will add an "Delete" button. You can find the savegames in "/steamapps/common/Eco/Eco_Data/Server/Storage", each savegame consists of a .db and a .eco file of the same name.

The delete button has long been implemented, just select a world in "Your Worlds" and press the X button in the top right area.