To switch between Seaside Farm and Island farm, you can simply tap the icon located on the left side of the Inbox. Depending on which farm you're located in, the icon will be different. When you tap on the icon, it will bring you to the other farm.

 

The left icon is for Seaside Farm and the right icon is for Island Farm:

 

or 

 

If you're on Seaside farm, you will see the icon on right side beside the Inbox icon. If you're on the Island farm, you will see the icon on the left side.