BAK file

Audacity crashed and my AUP file was replaced with a BAK file upon relaunch. How do I get my original file back?

Please check which program you are using. Audacity 2.x does not create BAK files.
You may have an old version of Audacity, Look in “Help menu > About Audacity” to confirm that it is Audacity and check the version number.
The current version of Audacity is 2.0.2 and it is available here: http://audacityteam.org/download/