First log into https://app.level365.com. Log in with your credentials, you must have elevated credentials to make this change. Contact support if you need to have your credentials elevated.
Click Manage Organization at the top right.
Click on the Music On Hold icon at the top.
Now click Add Music at the top right.
Now browse to the music on hold file you wish to upload. It needs to be either a .WAV or .MP3 file and not be copyrighted material. Type a name for the hold music in the "Song Name" field. Now press "Upload"
Once this sound file is uploaded it will be the new on-Hold and Park music for your domain.