If you want to relocate to a new island or maybe just make your main city more central in your hive, select the owned city that you would like to become the new main city, open the panel and click on the Main City Change button.

First Main City Change is free, afterwards it will cost 200 sapphires the first move and the price grows exponentially with this amount with each movement you make. You can also find free Main City Change items if you chose to run the items expedition.