When you are in a clan, you can see a list of players that are currently in your clan.
To the right of each player, you should see three stats. From left to right, these are Player Level, Trophies, and the category stat (this can be changed by tapping the arrows next to the category in the top-left).
You will also see two buttons in the top-right. There is a "Leave" button and a "3-Lines" button.
Tapping the "Leave" button will present an option for you to leave the clan. As it states on the confirmation prompt, any Clan Points that you have gained in the clan will stay with the clan when you leave and will not be carried over to any future clans that you may join. Here, you can tap the "Cancel" button to go back and stay in the current clan, or tap the "Leave" button to confirm leaving the clan.
Tapping the "3-Lines" button will present some options. These are Edit (if you are the leader or co-leader of the clan), Search, Leave, and Report.
Tapping Edit will take you to the Create A Clan screen where you can make changes to your clan.
Tapping the Search button will take you to a list of clans, you can search for clans by name by tapping the "Search for a clan..." box at the top of the screen.
Tapping the Leave button has the same behaviour as directly tapping the "Leave" button from before.
Tapping the Report Button brings up a menu with some checks. You can then check the related sections relative to your report. You will have to check the final section as a confirmation before being able to submit the report via the button in the bottom-right corner.