Leaving a Club is easy. Simply head to your Club tab, go to the top right corner, and click on the small "i" button. This will open up a pop-up in-game where you will be given the option to leave the Club. If you select the "leave" button, you will no longer be in that club. The trophies you earned with the club, before leaving, will remain there. Once you join a new club you will start at zero. Lifetime trophies are not affected during this process.