To leave a Project, click on the black circle icon in the Project header and then on "Leave project", or swipe left on the Project title in the Project tab and click the leave icon that appears on the left. 

This will not delete the Project and its contents for other members. However, if you are the only or last member of a Project, clicking the trash icon will delete it and it will not be retrievable.

You can display the list of the members of a Project by clicking the members icon in the top-right corner.