Switching from zsh to bash on OS X, and back again?
Solution 1:
You can just use exec to replace your current shell with a new shell:
Switch to bash:
exec bash
Switch to zsh:
exec zsh
This won't affect new terminal windows or anything, but it's convenient.
Solution 2:
you can try chsh -s /bin/bash to set the bash as the default,
or chsh -s /bin/zsh to set the zsh as the default.
Terminal will need a restart to take effect.