chsh -s $(which zsh)
You'll be prompted for your password, but once you update your settings any new iTerm/Terminal sessions you start on that machine will default to zsh.
In iTerm -> Preferences -> Profiles Tab -> General section set Command to: /bin/zsh --login
Go to the Users & Groups pane of the System Preferences -> Select the User -> Click the lock to make changes (bottom left corner) -> right click the current user select Advanced options... -> Select the Login Shell: /bin/zsh and OK
