(SPOILERS)
I got the bad end in chapter 3 where Kanon kills Echo, but I have no way of going back to choose to go with Missy and Forte. Is there any way to fix this?
(SPOILER)
If you have saved before choosing, you can go back by loading the save file. Otherwise:
This method activates developer mode. If you then want to disable it, just open "script.rpy" and change "define config.developer = True" to "define config.developer = False".