If you want to upload your audio recording to a podcast host, you may need an .mp3 file.
You can use the Editor to export your recording and select either an .mp3 or .wav file on the export options panel.
To convert audio files that you did not create on Riverside or that you edited in desktop editing software, use a program like Audacity. More information on how to do so is here: https://www.lifewire.com/audacity-tutorial-how-to-convert-wav-to-mp3-using-lame-2438749.