To leave the team to which you belong, open the team menu, then click on the "My Team" tab. You will then see a "Leave" button. After you confirm your choice, you will effectively leave the team.
If you leave a team of which you are the coach, your title is automatically transferred to the most senior member of your team. If you want another member to inherit your title, you will have to choose this person manually before you leave.
If you are the last member of your team, this team is automatically disbanded.