You can restart your sleep program at any time. Simply go to the Profile tab and scroll to the Settings section, then click on the "Restart Sleep Program" option. This will bring you back to the beginning (i.e. Day 1 of the program). You'll be asked to complete the onboarding quiz again, and you can choose a different program if you would like to.
This action is irreversible. You'll lose all saved progress. The only information that will be retained are your sleep logs and journal entries.