Instructions
Press the "M" key to open up the world map. Find Monolith Island in the southeast corner of the map screen.
Click on the southernmost red dot on Monolith Island. Press the "Negatus Statue" button on the left side of the screen after clicking the dot. You will then be transported to the island.
Speak with the Negatus statue by clicking on it once you reach Monolith Island. Progress through the dialog with the Negatus statue, clicking "More" each time it appears.
Press the "Leave Clan" button when it appears during the dialog with the Negatus statue. You will then leave your current clan and be free to join a new one.