Just open list of available worlds by clicking ‚Play in a new world that you haven’t discovered yet’ in World Selection window and choose one of them:
From time to time, when available worlds get crowded a new world is opened where all players can start their journey through the history from the beginning. We will make sure you are aware that a new world will be opened soon so keep an eye on our forums and announcements!
Remember: each world means starting the game from the beginning. You cannot move your progress, buildings or resources to any other world. The only exception are Diamonds! There is only one Diamond pool for all your worlds. Diamonds obtained on one world can be spent on another one, too!