To leave a team you are a member of, you can press the “Info” button and then press “Leave Team”. You’ll be able to find a new team to join by pressing the “Join Team” button, or by creating your own team.
If you are the owner of your team and you want to transfer the ownership of the team to another member, please make them an admin in the team. Once you have done so, make them a team admin again and your ownership will be transferred.