Hello Michelle @mhduong, thank you for your message.
Thanks a lot for choosing our product, it means a lot to us. 
The easiest way to start fresh is by creating a new app and then selecting Reviewly as a template from the field 'Start from a template (optional) '.
Bubble also has an option that will allow you to save your App at some point in the developing process, you can use the ‘Create save point’ button from the top right angle (Screenshot-bubble.io-2021.07.26-18_06_54.png - Nimbus Capture), please make sure you name the ‘Save points’ appropriately so you could easily remember them in case you would like to retrieve an old version of the app.
Let me know if you sorted it out.
Have a great rest of the day!