For iOS you have to edit
/var/mobile/Applications/<Application Identifier>/<Flappy Bird>/Library/Preferences/<Vendor>.<Bundle Identifier>.plist

And there should be a score tag you can edit, I recommend using iFile for this or iFunBox with Notepad++
You must have a jailbroken iDevice of course.

I tried to do it from memory because I don't have the app anymore.