From the app dashboard:
Press the downward arrow in the top left corner of the screen
Press the 'Manage Teams Icon'
Find the team you are looking to leave and press the leave icon (right hand side)
Press 'Leave Team'
From the web:
Go to the Dashboard
Click the Add/Join button in the upper left beside the team dropdown and select 'Manage teams'
Click Leave for any team you no longer want to be apart of