On OS X, how do I change my shell from fish back to bash?

Very easy:

• from FISH to BASH :
  In your Fish terminal, type bash --login to switch to Bash
• from BASH to FISH :
  In your Bash terminal, type fish to switch to Fish

Go to System Preferences, Users & Groups, click the lock to make changes, right click (or Control click) on your username, choose "Advanced Options" and you should have a field to change your shell. Change it there, reboot, and your new shell should take effect.