To leave a Community, simply go to the main Chat window of the Community and click on "Profile", where you will be able to see the information of the team. In this window, there is a red button at the bottom where you can click to leave your Community.

 Please, keep in mind that, if you're the Founder, you won't be able to leave if there are other members. In this case, you will need to delete all members from the Community before you leave.