How to convert YouTube offline exo file to MP4

You can't, exo is compressed and encrypted so only the same youtube App that you used to save it can obtain the video stream content. The encryption key is specific for the youtube app installation and change from device to device in order to avoid that once you downloaded the content you can distribute it for play on another device. Ideally if you have a rooted Android device you should find somewhere within the youtube application data the encryption key but then you will not have the required info on how to use that to decrypt the .exo which very unlikely Google will share since with that knowledge its copyright protection system would not be safe.