That's why it's always useful to have an Undo button at the ready, because it's nice to be able to quickly check how new ideas work without impeding on your creativity. Sometimes it's even useful to be able to start from scratch.
Here are two methods that will help you undo changes that you've made on your design or even scrap it completely and start anew:
A. Undoing a change
1. Click on the Undo button, symbolized with ↺ on the menu located at the top left of the Form Designer page.
You can undo as many changes as you want but bear in mind that undoing a change means you cannot redo it automatically. You'll have to redo everything manually if you change your mind!
B. Starting from scratch
1. Click the Clear button on the menu located at the top of the Form Designer page.
2. Click on the OK button on the pop-up message that appears.
A generally good idea is to work with drafts inside the Form Designer, so that you always feel safe in the knowledge that your work won't get lost. You'll always be able to use one of your drafts as a fallback whenever you change your mind!