To leave your current clan you'll have to be in the clan-home menu, and the click on the clan-settings (the three dots). When you are in the clan-settings, there will be a "leave clan" option at the top. If you press this, the game will ask you "are you sure you want to leave the clan". If you press yes, you will then proceed to leave the clan.
! If you are the leader of the clan, you will have to promote someone else to leader, before you will be able to leave the clan. Also, if you are the only person in the clan and leave the clan, you will not be able to join the clan again.