Harbors are located on islands around the world. Each Harbor has its own unique number.
The harbor is a central hub for all your inventory manipulation. To enter the harbor, you have to get close enough. Once you are, an “enter harbor” button will appear on the top right of the screen.
To swap items:
1) Enter the harbor and select the items tab.
2) Unselect the item that needs to be swapped (in the “Sailing” box). After unselecting, the item should appear in the box on the right (“In harbor” box)
3) Select the item you want to take with you on your ship (from the “In harbor” box). After selecting it should appear in the left box (“Sailing” box).
4) The last step is to approve the transaction.